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Algebra II math worksheets

Build your own math worksheets from 35,000+ problems for grades 3 to 12, from fractions to AP Calculus. Every problem comes with step-by-step solutions.

Quadratic forms and transformations

Learning goals

  • Converts among useful forms of a quadratic when appropriate.
  • Identifies vertex, axis of symmetry, and intercepts from the relevant form.
  • Describes graph transformations from parameters.
  • Selects the form best suited to a given question.

Click problems to add them to your worksheet.

Quadratic forms and transformations 1
The graph of \(g\) is a translation of \(f(x)=x^2\). Describe the translation and write \(g(x)\).
Figure for problem 533580

Hints

- Compare the positions of the vertices. - A horizontal shift changes the expression inside the square. - A vertical shift changes the term outside the square.

Solution

1. The vertex of \(f\) is \((0, 0)\), while the vertex of \(g\) is \((-2, -2)\). 2. The shift is left \(2\) and down \(2\). 3. Therefore, \(g(x)=(x+2)^2-2\).

Answer

Shift left \(2\) units and down \(2\) units; \(g(x)=(x+2)^2-2\)
Quadratic forms and transformations 2
The red graph \(g\) is a transformation of the blue graph \(f(x)=x^2-2\). Describe the transformation and write \(g(x)\) in terms of \(f(x)\).
Figure for problem 534092

Hints

- Compare the opening directions of the two parabolas. - Check whether corresponding y-values have opposite signs. - A reflection across the x-axis multiplies all outputs by \(-1\).

Solution

1. The two parabolas have the same width and the same axis of symmetry. 2. Every point \((x, y)\) on \(f\) corresponds to \((x, -y)\) on \(g\). For example, \(f(0)=-2\) and \(g(0)=2\). 3. This is a reflection across the x-axis, so \(g(x)=-f(x)\).

Answer

Reflect across the x-axis; \(g(x)=-f(x)\)
Quadratic forms and transformations 3
Consider \(f(x)=2x^2-6x+5\). Without rewriting the function, state the y-intercept and whether the parabola opens upward or downward.

Hints

- Which coefficient can be read directly when \(x=0\)? - What does the sign of the coefficient of \(x^2\) tell you about the opening direction? - Use only information visible in the given standard form.

Solution

1. In standard form \(ax^2+bx+c\), the constant term gives \(f(0)\). Here \(f(0)=5\), so the y-intercept is \((0,5)\). 2. The leading coefficient is \(2>0\), so the parabola opens upward.

Answer

The y-intercept is \((0,5)\), and the parabola opens upward.
Quadratic forms and transformations 4
Consider \(g(x)=-3(x+2)^2+7\). State the vertex, the axis of symmetry, and whether the vertex is a maximum or a minimum.

Hints

- Compare the expression with \(a(x-h)^2+k\). - Be careful with the sign inside the squared binomial. - Use the sign of the outside coefficient to decide maximum or minimum.

Solution

1. Vertex form is \(a(x-h)^2+k\). Here \(h=-2\) and \(k=7\), so the vertex is \((-2,7)\). 2. The axis of symmetry is \(x=-2\). 3. Since \(a=-3<0\), the parabola opens downward, so the vertex is a maximum.

Answer

Vertex: \((-2,7)\) Axis of symmetry: \(x=-2\) The vertex is a maximum.
Quadratic forms and transformations 5
Consider \(h(x)=0.5(x-4)(x+1)\). State the zeros, the x-intercepts, and whether the parabola opens upward or downward.

Hints

- In factored form, when does the function equal zero? - Convert each zero into an ordered pair on the x-axis. - Check the sign of the overall scale factor.

Solution

1. The factors are zero when \(x=4\) or \(x=-1\), so the zeros are \(4\) and \(-1\). 2. The x-intercepts are \((4,0)\) and \((-1,0)\). 3. The leading coefficient is positive, so the parabola opens upward.

Answer

Zeros: \(x=-1\) and \(x=4\) x-intercepts: \((-1,0)\) and \((4,0)\) The parabola opens upward.
Quadratic forms and transformations 6
Fill the blanks in \(f(x)=a(x-\square)^2+\square\) using the graph. You do not need to find \(a\).
Figure for problem 560707

Hints

- Where is the lowest point of the curve? - Which vertex coordinate belongs inside the square, and which belongs outside?

Solution

1. The vertex is \((2,-3)\). 2. Vertex form is \(a(x-h)^2+k\), where \((h,k)\) is the vertex. 3. Therefore, the blanks are \(2\) and \(-3\).

Answer

\(2\), then \(-3\); \(f(x)=a(x-2)^2-3\)
Quadratic forms and transformations 7
A parabola crosses the x-axis at \(x=-6\) and \(x=2\). What is the equation of its axis of symmetry?

Hints

- The two x-intercepts are the same distance from the axis of symmetry. - Find the midpoint of their x-coordinates.

Solution

1. The axis of symmetry lies halfway between the two zeros. 2. Their midpoint is \(\frac{-6+2}{2}=-2\). 3. The axis is \(x=-2\).

Answer

\(x=-2\)
Quadratic forms and transformations 8
To shift \(f(x)=x^2\) down \(5\) units, should you use \(g(x)=x^2-5\) or \(h(x)=(x-5)^2\)? Explain.

Hints

- A vertical shift changes function outputs, not inputs. - Compare what the two rules do to the vertex \((0,0)\).

Solution

1. Shifting down changes every output by \(-5\). 2. Thus \(g(x)=f(x)-5=x^2-5\) is correct; \(h(x)\) shifts the graph right \(5\) units.

Answer

Use \(g(x)=x^2-5\), because the subtraction is outside the square and moves the graph down.
Quadratic forms and transformations 9
The graph of the parent function \(y = x^2\) is vertically stretched by a factor of \(3\), shifted up \(1\) unit, and then shifted right \(2\) units. Which equation represents the transformed graph? a) \(y = 3(x+2)^2 + 1\) b) \(y = 3(x-2)^2 + 1\) c) \(y = 3[(x+2)^2 + 1]\) d) \(y = 3[(x-2)^2 + 1]\)

Hints

- How do vertical and horizontal shifts change a function rule? - For a shift to the right, decide whether the number inside the input should be added or subtracted. - Track which operations affect the entire function value and which affect the input.

Solution

1. A vertical stretch by a factor of \(3\) gives \(y = 3x^2\). 2. Shifting up \(1\) unit gives \(y = 3x^2 + 1\). 3. Shifting right \(2\) units replaces \(x\) with \(x-2\), so \(y = 3(x-2)^2 + 1\).

Answer

b) \(y = 3(x-2)^2 + 1\)
Quadratic forms and transformations 10
For each function \(f\), write rules for \(g\) and \(h\), where the graph of \(g\) is the reflection of the graph of \(f\) across the x-axis and the graph of \(h\) is the reflection across the y-axis. a) \(f(x)=x^2-4x\) b) \(f(x)=\frac{1}{2}(x+2)^2-3\)

Hints

- A reflection across the x-axis changes every output to its opposite. - A reflection across the y-axis replaces each input \(x\) with \(-x\). - Use parentheses when substituting \(-x\), especially inside powers. - Simplify each resulting rule when possible.

Solution

1. A reflection across the x-axis is given by \(g(x)=-f(x)\). 2. For part a, \(g(x)=-(x^2-4x)=-x^2+4x\). 3. For part b, \(g(x)=-\left[\frac{1}{2}(x+2)^2-3\right]= -\frac{1}{2}(x+2)^2+3\). 4. A reflection across the y-axis is given by \(h(x)=f(-x)\). 5. For part a, \(h(x)=(-x)^2-4(-x)=x^2+4x\). 6. For part b, \(h(x)=\frac{1}{2}(-x+2)^2-3=\frac{1}{2}(x-2)^2-3\).

Answer

a) \(g(x)=-x^2+4x\); \(h(x)=x^2+4x\) b) \(g(x)=-\frac{1}{2}(x+2)^2+3\); \(h(x)=\frac{1}{2}(x-2)^2-3\)
Quadratic forms and transformations 11
The graph of the parent function \(f(x)=x^2\) has vertex \((0, 0)\). For each transformed function, state the vertex and describe the transformations. a) \(g(x)=(x+4)^2-2\) b) \(h(x)=3(x-5)^2+6\) c) \(k(x)=-x^2+4\)

Hints

- In vertex form, identify the horizontal and vertical shifts. - Replacing \(x\) with \(x-c\) moves the vertex horizontally. - A constant outside the squared expression moves the vertex vertically. - A vertical scale factor does not move a vertex that lies on the horizontal shift line.

Solution

1. For \(g(x)=(x+4)^2-2\), the graph shifts left \(4\) units and down \(2\) units, so the vertex is \((-4, -2)\). 2. For \(h(x)=3(x-5)^2+6\), the graph shifts right \(5\) units and up \(6\) units. The vertical stretch does not change the translated vertex, so the vertex is \((5, 6)\). 3. For \(k(x)=-x^2+4\), the graph reflects across the x-axis and shifts up \(4\) units. The vertex is \((0, 4)\).

Answer

a) Vertex: \((-4, -2)\); shift left \(4\) units and down \(2\) units b) Vertex: \((5, 6)\); shift right \(5\) units, vertically stretch by a factor of \(3\), and shift up \(6\) units c) Vertex: \((0, 4)\); reflect across the x-axis and shift up \(4\) units
Quadratic forms and transformations 12
A quadratic function \(f\) has a local maximum at \((2, 6)\). For each transformed function, state the coordinates and type of the corresponding vertex. a) \(g(x)=f(x)-8\) b) \(h(x)=f(x+3)\) c) \(k(x)=-2f(x)\)

Hints

- Decide whether each transformation changes inputs or outputs. - Track the coordinates of the given vertex. - A negative outside coefficient reflects the graph across the x-axis. - A reflection across the x-axis switches maxima and minima.

Solution

1. The rule \(g(x)=f(x)-8\) shifts the graph down \(8\) units. The maximum moves to \((2, -2)\). 2. The rule \(h(x)=f(x+3)\) shifts the graph left \(3\) units. The maximum moves to \((-1, 6)\). 3. The rule \(k(x)=-2f(x)\) vertically stretches by a factor of \(2\) and reflects across the x-axis. The point \((2, 6)\) maps to \((2, -12)\), and the maximum becomes a minimum.

Answer

a) Maximum at \((2, -2)\) b) Maximum at \((-1, 6)\) c) Minimum at \((2, -12)\)
Quadratic forms and transformations 13
Let \(f(x)=x^2-2x\) and \(g(x)=-3(x^2-2x)+4\). Describe, in order, how to obtain the graph of \(g\) from the graph of \(f\).

Hints

- Recognize \(f(x)\) as a complete expression inside the rule for \(g\). - An outside factor changes all outputs. - A constant added outside the function creates a vertical shift. - Pay attention to the sign of the scale factor.

Solution

1. Since \(g(x)=-3f(x)+4\), multiply every output of \(f\) by \(-3\). 2. This reflects the graph across the x-axis and vertically stretches it by a factor of \(3\). 3. Then shift the result up \(4\) units.

Answer

Reflect across the x-axis, vertically stretch by a factor of \(3\), and then shift up \(4\) units.
Quadratic forms and transformations 14
The path of water from a fountain is modeled by \(h(d)=-0.5d^2+2d+1\), where \(d\) is the horizontal distance from the nozzle and \(h(d)\) is the height above the ground, both in feet. The nozzle is mounted on a \(2.5\)-foot platform without changing the shape of the water path. a) Write the new model \(h_{\mathrm{new}}(d)\). b) Describe the transformation from the graph of \(h\) to the graph of \(h_{\mathrm{new}}\). c) The original vertex is \((2, 3)\). State the new vertex and justify your answer.

Hints

- Raising the starting position adds the same amount to every height. - A vertical shift does not change the horizontal location of the maximum. - Track how \(f(x)+c\) changes the coordinates of a point.

Solution

1. Raising the nozzle by \(2.5\) feet adds \(2.5\) to every height: \(h_{\mathrm{new}}(d)=h(d)+2.5=-0.5d^2+2d+3.5\). 2. This is a vertical shift up \(2.5\) units. 3. A vertical shift does not change the horizontal coordinate of the vertex. The vertical coordinate increases by \(2.5\), so \((2, 3)\) moves to \((2, 5.5)\).

Answer

a) \(h_{\mathrm{new}}(d)=-0.5d^2+2d+3.5\) b) Shift the graph up \(2.5\) units. c) The new vertex is \((2, 5.5)\).
Quadratic forms and transformations 15
Let \(f(x)=(x+5)^2-2\). Show algebraically that the graph is symmetric about the vertical line \(x=-5\).

Hints

- Use inputs equally far from the proposed vertical axis. - Substitute \(-5+h\) and \(-5-h\). - Compare the resulting squared expressions.

Solution

1. Symmetry about \(x=-5\) requires equal outputs at inputs equally far from \(-5\). 2. \(f(-5+h)=h^2-2\). 3. \(f(-5-h)=(-h)^2-2=h^2-2\). 4. Therefore, \(f(-5+h)=f(-5-h)\) for every real \(h\), proving symmetry about \(x=-5\).

Answer

Both \(f(-5+h)\) and \(f(-5-h)\) simplify to \(h^2-2\). Therefore, the graph is symmetric about \(x=-5\).
Quadratic forms and transformations 16
The parent function is \(f(x)=x^2\). The point \(V\) marks the vertex of \(g\). a) Describe the reflection and shifts that transform \(f\) into \(g\). b) Write a rule for \(g\).
Figure for problem 532163

Hints

- Compare the vertices of the two parabolas. - Check whether they open in the same direction. - Use vertex form to express the reflection and shifts.

Solution

1. The vertex moves from \((0, 0)\) to \((2, 1)\), a shift right \(2\) and up \(1\). 2. The transformed parabola opens downward, so reflect across the x-axis. 3. Therefore, \(g(x)=-(x-2)^2+1\).

Answer

a) Reflect across the x-axis, shift right \(2\) units, and shift up \(1\) unit. b) \(g(x)=-(x-2)^2+1\)
Quadratic forms and transformations 17
The parent function is \(f(x)=x^2\). The graphs \(g\) and \(h\) are obtained by shifts and possibly a reflection. Find rules for \(g\) and \(h\).
Figure for problem 532462

Hints

- Read the vertex of each parabola. - Decide whether each curve opens up or down. - Compare their widths with the parent parabola before choosing the coefficient.

Solution

1. The vertex of \(g\) is \((2, -3)\); it opens upward with the same width as \(f\). Thus \(g(x)=(x-2)^2-3\). 2. Expanding gives \(g(x)=x^2-4x+1\). 3. The vertex of \(h\) is \((-3, 4)\); it opens downward with the same width as \(f\). Thus \(h(x)=-(x+3)^2+4\). 4. Expanding gives \(h(x)=-x^2-6x-5\).

Answer

\(g(x)=(x-2)^2-3\), or \(g(x)=x^2-4x+1\) \(h(x)=-(x+3)^2+4\), or \(h(x)=-x^2-6x-5\)
Quadratic forms and transformations 18
The parent function is \(f(x)=x^2\). The functions \(g\) and \(h\) are translations of \(f\). Write the rules for \(g\) and \(h\).
Figure for problem 534047

Hints

- Compare each vertex with the parent's vertex. - What expression inside a square becomes zero at each new vertex? - Substitute each vertex's x-coordinate into your rule to check its height.

Solution

1. The vertex of \(g\) is \((4, -1)\), so \(g(x)=(x-4)^2-1\). 2. The vertex of \(h\) is \((-3, -1)\), so \(h(x)=(x+3)^2-1\).

Answer

\(g(x)=(x-4)^2-1\) \(h(x)=(x+3)^2-1\)
Quadratic forms and transformations 19
The blue graph is \(f(x)=\frac{1}{4}x^2-1\). In panels a) through c), each red graph is a transformation of \(f\). Match each red graph to one rule and describe the transformation. 1. \(g(x)=f(x-2)\) 2. \(h(x)=f(x)+1.5\) 3. \(k(x)=-f(x)\)
Figure for problem 534061

Hints

- Compare the vertices of the blue and red graphs. - Decide whether the change affects x-coordinates or y-coordinates. - A vertical shift preserves the shape and opening direction. - A reflection across the x-axis reverses the opening direction.

Solution

1. In panel a), the vertex moves from \((0, -1)\) to \((2, -1)\), so the graph shifts right \(2\) units. This matches \(g(x)=f(x-2)\). 2. In panel b), the vertex moves from \((0, -1)\) to \((0, 0.5)\), so the graph shifts up \(1.5\) units. This matches \(h(x)=f(x)+1.5\). 3. In panel c), the vertex moves from \((0, -1)\) to \((0, 1)\), and the parabola opens downward. This is a reflection across the x-axis and matches \(k(x)=-f(x)\).

Answer

a) \(g(x)=f(x-2)\), shift right \(2\) units; b) \(h(x)=f(x)+1.5\), shift up \(1.5\) units; c) \(k(x)=-f(x)\), reflect across the x-axis.
Quadratic forms and transformations 20
Let \(f(x)=x^2\), and consider the transformation \(g(x)=af(x+c)+d\), where \(a\ne0\). Explain the effect of each parameter \(a\), \(c\), and \(d\) on the graph of \(f\), including the effect of a negative value of \(a\). Then identify the parameter values shown in the graph.
Figure for problem 534063

Hints

- Separate changes to the output from changes to the input. - Consider both the magnitude and sign of \(a\), using the stated restriction on \(a\). - Changes inside the input act in the opposite horizontal direction. - Use the transformed vertex to identify the example values.

Solution

1. Because \(a\ne0\), the parameter \(a\) scales all y-values by \(|a|\). If \(|a|>1\), the graph is stretched vertically; if \(0<|a|<1\), it is compressed vertically. If \(a<0\), the graph is also reflected across the x-axis. 2. The parameter \(c\) controls the horizontal shift. In \(f(x+c)\), a positive \(c\) shifts the graph left, while a negative \(c\) shifts it right. 3. The parameter \(d\) controls the vertical shift. A positive \(d\) shifts the graph up, while a negative \(d\) shifts it down. 4. In the graph, the parabola is reflected across the x-axis, shifted right \(2\) units, and shifted up \(1\) unit. Therefore, \(a=-1\), \(c=-2\), and \(d=1\).

Answer

For \(a\ne0\): \(a\): vertical scale by \(|a|\), with reflection across the x-axis when \(a<0\) \(c\): horizontal shift left for \(c>0\) and right for \(c<0\) \(d\): vertical shift up for \(d>0\) and down for \(d<0\) Shown example: \(a=-1\), \(c=-2\), \(d=1\)
Quadratic forms and transformations 21
The parent function is \(f(x)=(x+1)^2\). Write the transformed function as \(g(x)=f(x+c)+d\). Find \(c\) and \(d\) from the vertices.
Figure for problem 534065

Hints

- Locate both vertices. - Find their horizontal and vertical changes. - Test the sign of \(c\): at the new vertex's x-coordinate, does \(x+c\) equal the old vertex's x-coordinate?

Solution

1. The vertices are \((-1, 0)\) for \(f\) and \((1, 1)\) for \(g\). 2. The graph moves right \(2\), which gives \(c=-2\), and up \(1\), which gives \(d=1\). 3. Therefore, \(g(x)=f(x-2)+1\).

Answer

\(c=-2\), \(d=1\), and \(g(x)=f(x-2)+1\).
Quadratic forms and transformations 22
The graph of \(g\) is a translation of \(f\). Describe the translation and write \(g(x)\) in terms of \(f(x)\).
Figure for problem 534071

Hints

- Compare the vertices of the graphs. - Check whether either vertex changes height. - Write a shift to the right using subtraction in the input.

Solution

1. The vertex moves from \((-2,-2)\) to \((3,-2)\). 2. This is a shift right \(5\) units, with no vertical change. 3. Therefore, \(g(x)=f(x-5)\).

Answer

Shift right \(5\) units; \(g(x)=f(x-5)\)
Quadratic forms and transformations 23
The red graph \(g\) is a horizontal translation of the blue graph \(f(x)=x(x-2)\). Determine the translation and write \(g(x)\) in terms of \(f(x)\).
Figure for problem 534072

Hints

- Compare the zeros of the two graphs. - Check that both zeros move by the same amount. - A shift left uses addition inside the input.

Solution

1. The zeros of \(f\) are \(x=0\) and \(x=2\). The corresponding zeros of \(g\) are \(x=-4\) and \(x=-2\). 2. Each zero moves left \(4\) units, so the entire graph shifts left \(4\) units. 3. A shift left \(4\) units replaces \(x\) with \(x+4\), so \(g(x)=f(x+4)\).

Answer

Shift left \(4\) units; \(g(x)=f(x+4)\)
Quadratic forms and transformations 24
The blue graph is an upward-opening parabola \(f\), and the red graph is \(g\). Describe the reflection and scaling that produce the red graph. Write \(g(x)\) in terms of \(f(x)\).
Figure for problem 534075

Hints

- Compare the opening directions. - Compare the y-values of the two graphs at \(x=0\). - Separate the sign of the scale factor from its magnitude.

Solution

1. The blue graph opens upward, and the red graph opens downward, so the graph is reflected across the x-axis. 2. At \(x=0\), the graph shows \(f(0)=-4\) and \(g(0)=2\). Therefore, the output is multiplied by \(-\frac{1}{2}\), because \(-\frac{1}{2}(-4)=2\). 3. The factor \(\frac{1}{2}\) is a vertical compression, and the negative sign represents the reflection. 4. Thus, \(g(x)=-\frac{1}{2}f(x)\).

Answer

Reflect across the x-axis and vertically compress by a factor of \(\frac{1}{2}\); \(g(x)=-\frac{1}{2}f(x)\)
Quadratic forms and transformations 25
The red graph \(g\) is a translation of the blue graph \(f\). Describe the translation and write \(g(x)\) in terms of \(f(x)\).
Figure for problem 534091

Hints

- Compare the vertices of the two parabolas. - Check whether the shape or opening direction changed. - Horizontal shifts affect the input, and vertical shifts affect the output.

Solution

1. The vertex of \(f\) is \((0, -2)\), and the vertex of \(g\) is \((3, -1)\). 2. The vertex moves right \(3\) units and up \(1\) unit. 3. Therefore, \(g(x)=f(x-3)+1\).

Answer

Shift right \(3\) units and up \(1\) unit; \(g(x)=f(x-3)+1\)
Quadratic forms and transformations 26
The red graph \(g\) is a transformation of the blue graph \(f\). Describe the transformation and write \(g(x)\) in terms of \(f(x)\).
Figure for problem 534096

Hints

- Compare the vertices relative to the y-axis. - Look for corresponding points with opposite x-coordinates. - A y-axis reflection replaces \(x\) with \(-x\).

Solution

1. The vertex of \(f\) is \((2, -1)\), while the vertex of \(g\) is \((-2, -1)\). 2. Corresponding points have opposite x-coordinates and equal y-coordinates. For example, the zero at \(x=3\) on \(f\) corresponds to the zero at \(x=-3\) on \(g\). 3. This is a reflection across the y-axis, which replaces \(x\) with \(-x\). 4. Therefore, \(g(x)=f(-x)\).

Answer

Reflect across the y-axis; \(g(x)=f(-x)\)
Quadratic forms and transformations 27
The graph has the form \(g(x)=a(x+b)^2+c\). Which sign pattern matches the graph? 1) \(a<0\), \(b<0\), \(c<0\) 2) \(a<0\), \(b>0\), \(c>0\) 3) \(a>0\), \(b<0\), \(c>0\) 4) \(a>0\), \(b>0\), \(c<0\)
Figure for problem 535021

Hints

- Use the vertex \((-b, c)\) to determine the signs of \(b\) and \(c\). - Use the opening direction to determine the sign of \(a\). - Check each sign separately before choosing an option.

Solution

1. The vertex is \((-2, 3)\). 2. For \(g(x)=a(x+b)^2+c\), the vertex is \((-b, c)\). Since \(-b=-2\), \(b=2>0\). Since the vertex y-coordinate is \(3\), \(c=3>0\). 3. The parabola opens downward, so \(a<0\). 4. Therefore, the correct sign pattern is option 2.

Answer

2) \(a<0\), \(b>0\), \(c>0\)
Quadratic forms and transformations 28
Rewrite \(f(x)=x^2-8x+11\) in vertex form. Then use that form to state the minimum value of \(f\) and the x-value where it occurs.

Hints

- Which equivalent quadratic form makes a minimum or maximum visible immediately? - What number would make the first three terms a perfect-square trinomial? - After rewriting, use the smallest possible value of the squared expression.

Solution

1. Rewrite the quadratic as \(x^2-8x+16-16+11\). 2. Then \(f(x)=(x-4)^2-5\). 3. The squared term is at least \(0\), so the minimum value is \(-5\) when \(x=4\).

Answer

\(f(x)=(x-4)^2-5\). The minimum value is \(-5\) at \(x=4\).
Quadratic forms and transformations 29
Rewrite \(f(x)=2x^2+x-6\) in factored form. Then use that form to state the x-intercepts of the graph.

Hints

- Which equivalent form makes zeros visible directly? - Look for two binomial factors whose product reproduces all three terms. - Set each factor equal to zero only after the factorization is complete.

Solution

1. Factor the quadratic: \(2x^2+x-6=(2x-3)(x+2)\). 2. The factors are zero when \(x=\frac{3}{2}\) or \(x=-2\). 3. Therefore, the x-intercepts are \(\left(\frac{3}{2},0\right)\) and \((-2,0)\).

Answer

\(f(x)=(2x-3)(x+2)\). The x-intercepts are \((-2,0)\) and \(\left(\frac{3}{2},0\right)\).
Quadratic forms and transformations 30
The same quadratic is written in three equivalent forms: \(f(x)=x^2-6x+5\), \(f(x)=(x-1)(x-5)\), \(f(x)=(x-3)^2-4\). a) Which form is most useful for finding the zeros? State the zeros. b) Which form is most useful for finding the minimum? State the minimum value and where it occurs. c) Which form is most useful for finding the y-intercept? State the y-intercept.

Hints

- Ask which feature is visible without additional algebra in each form. - Zeros come from inputs that make a product equal zero. - A squared expression shifted vertically makes an extremum easy to read. - The y-intercept occurs when \(x=0\).

Solution

1. Factored form shows the zeros directly: \(x=1\) and \(x=5\). 2. Vertex form shows the minimum directly: the minimum value is \(-4\) at \(x=3\). 3. Standard form shows \(f(0)=5\), so the y-intercept is \((0,5)\).

Answer

a) Factored form; zeros \(x=1\) and \(x=5\) b) Vertex form; minimum \(-4\) at \(x=3\) c) Standard form; y-intercept \((0,5)\)
Quadratic forms and transformations 31
Expand \(f(x)=(x+3)^2-4\) into standard form \(ax^2+bx+c\).

Hints

- Expand the squared binomial before combining constants. - Check the middle term; the two cross-products both matter.

Solution

1. Expand the square: \((x+3)^2=x^2+6x+9\). 2. Subtract \(4\): \(f(x)=x^2+6x+5\).

Answer

\(f(x)=x^2+6x+5\)
Quadratic forms and transformations 32
Write \(f(x)=-2(x-3)(x+1)\) in standard form.

Hints

- Expand the two binomial factors first. - Then apply the coefficient outside the product to every term. - Check the sign of the leading coefficient in the result.

Solution

1. Multiply the binomials: \((x-3)(x+1)=x^2-2x-3\). 2. Multiply by \(-2\): \(f(x)=-2x^2+4x+6\).

Answer

\(f(x)=-2x^2+4x+6\)
Quadratic forms and transformations 33
A quadratic has zeros at \(x=2\) and \(x=6\). Its y-intercept is \((0,24)\). Write the rule in factored form.

Hints

- Each zero gives a factor that becomes zero at that input. - Use the y-intercept to find the remaining scale factor. - Substitute that value into the factored rule to check it.

Solution

1. The zeros give \(f(x)=a(x-2)(x-6)\). 2. Using the y-intercept: \(24=a(-2)(-6)=12a\), so \(a=2\). 3. Hence \(f(x)=2(x-2)(x-6)\).

Answer

\(f(x)=2(x-2)(x-6)\)
Quadratic forms and transformations 34
A quadratic has vertex \((1,-4)\) and passes through \((3,4)\). Write its equation in vertex form.

Hints

- What information does the vertex give you about the two shifts? - Use the other point to determine the coefficient outside the square. - Check both given points with your final rule.

Solution

1. With vertex \((1,-4)\), write \(f(x)=a(x-1)^2-4\). 2. Substitute \((3,4)\): \(4=a(3-1)^2-4\), so \(8=4a\) and \(a=2\). 3. Therefore, \(f(x)=2(x-1)^2-4\).

Answer

\(f(x)=2(x-1)^2-4\)
Quadratic forms and transformations 35
Let \(f(x)=x^2\). Rewrite \(g(x)=-0.5x^2+2x+1\) in the form \(g(x)=af(x+c)+d\). Then describe, in order, the transformations that produce the graph of \(g\) from the graph of \(f\).

Hints

- What feature of the rewritten quadratic will make the horizontal and vertical shifts visible? - Can you rewrite the quadratic so one squared binomial is visible? - After rewriting, compare each part with \(af(x+c)+d\), paying close attention to signs.

Solution

1. Complete the square: \(g(x)=-0.5(x^2-4x)+1\). 2. Since \(x^2-4x=(x-2)^2-4\), \(g(x)=-0.5[(x-2)^2-4]+1=-0.5(x-2)^2+3\). 3. Therefore, \(g(x)=-0.5f(x-2)+3\), so \(a=-0.5\), \(c=-2\), and \(d=3\). 4. Starting with the graph of \(f\), shift right \(2\) units, vertically compress by a factor of \(0.5\) and reflect across the x-axis, then shift up \(3\) units.

Answer

\(g(x)=-0.5f(x-2)+3\). Transformations: shift right \(2\) units; vertically compress by a factor of \(0.5\) and reflect across the x-axis; shift up \(3\) units.
Quadratic forms and transformations 36
The graph of \(f(x)=\frac{1}{2}(x-2)^2-4\) is transformed in this order: 1. Reflect the graph across the x-axis. 2. Shift the result left \(2\) units. Write the rule for the resulting function \(g\) first in vertex form and then in the form \(g(x)=ax^2+c\).

Hints

- A reflection across the x-axis changes the sign of the entire output. - For a horizontal shift, think about how the input of the function must change. - Apply the transformations in the stated order. - Track the vertex after each transformation.

Solution

1. Reflecting across the x-axis multiplies all outputs by \(-1\): \(f_1(x)=-\left[\frac{1}{2}(x-2)^2-4\right]= -\frac{1}{2}(x-2)^2+4\). 2. Shifting left \(2\) units replaces \(x\) with \(x+2\): \(g(x)=-\frac{1}{2}[(x+2)-2]^2+4=-\frac{1}{2}x^2+4\). 3. The vertex form is \(g(x)=-\frac{1}{2}(x-0)^2+4\), which simplifies to \(g(x)=-\frac{1}{2}x^2+4\).

Answer

Vertex form: \(g(x)=-\frac{1}{2}(x-0)^2+4\) Standard form: \(g(x)=-\frac{1}{2}x^2+4\)
Quadratic forms and transformations 37
The graph of \(g(x)=x^2-9x+21\) is obtained from the graph of \(f(x)=x^2-3x\) by shifting right by \(c\) units and up by \(d\) units. Find \(c\) and \(d\).

Hints

- A translation moves corresponding graph features by the same horizontal and vertical amounts. - Which feature of each parabola could help you determine the horizontal shift first? - After finding one shift, use another corresponding feature or the equations to determine the other. - Check your values by confirming that the transformed quadratic matches \(g\).

Solution

1. A shift right by \(c\) units and up by \(d\) units gives \(g(x)=f(x-c)+d\). 2. Substitute the rule for \(f\): \(g(x)=(x-c)^2-3(x-c)+d\). 3. Expand: \(g(x)=x^2+(-2c-3)x+(c^2+3c+d)\). 4. Compare the coefficient of \(x\): \(-2c-3=-9\), so \(c=3\). 5. Compare constants: \(c^2+3c+d=21\). Substituting \(c=3\) gives \(9+9+d=21\), so \(d=3\).

Answer

\(c=3\) and \(d=3\)
Quadratic forms and transformations 38
The function \(g(x)=0.5(x+3)^2-4\) is obtained from the parent function \(f(x)=x^2\). Determine which transformation sequences correctly produce \(g\). Sequence 1: Shift left \(3\) units, shift down \(8\) units, then vertically compress by a factor of \(0.5\). Sequence 2: Shift left \(3\) units, vertically compress by a factor of \(0.5\), then shift down \(8\) units. Sequence 3: Vertically compress by a factor of \(0.5\), shift left \(3\) units, then shift down \(4\) units.

Hints

- Apply each transformation to the current function rule, one step at a time. - A vertical compression applied after a vertical shift also scales that shift. - Horizontal shifts commute with transformations that change only outputs.

Solution

1. Sequence 1 gives \((x+3)^2\), then \((x+3)^2-8\), then \(0.5[(x+3)^2-8]=0.5(x+3)^2-4\). It is correct. 2. Sequence 2 gives \((x+3)^2\), then \(0.5(x+3)^2\), then \(0.5(x+3)^2-8\). It is not correct. 3. Sequence 3 gives \(0.5x^2\), then \(0.5(x+3)^2\), then \(0.5(x+3)^2-4\). It is correct.

Answer

Sequences 1 and 3 are correct. Sequence 2 produces \(y=0.5(x+3)^2-8\), not \(g\).
Quadratic forms and transformations 39
Let \(f(x)=x^2\) and \(g(x)=0.5x^2-3\). Lina says, “First vertically compress the graph of \(f\) by a factor of \(0.5\), then shift it down \(3\) units.” Felix says, “First shift the graph of \(f\) down \(3\) units, then vertically compress it by a factor of \(0.5\).” Determine whether each description produces the graph of \(g\).

Hints

- Write the function rule after each individual transformation. - Decide whether the scale factor applies to only \(x^2\) or to the entire current output. - Multiplying and then subtracting generally differs from subtracting and then multiplying. - Compare each final rule with \(g(x)\).

Solution

1. Lina first obtains \(0.5x^2\), then shifts down \(3\) units: \(g_L(x)=0.5x^2-3\). Her description produces \(g\). 2. Felix first obtains \(x^2-3\), then compresses the entire output: \(g_F(x)=0.5(x^2-3)=0.5x^2-1.5\). 3. Since \(g_F(x)\ne g(x)\), Felix’s description does not produce \(g\).

Answer

Only Lina’s description produces \(g(x)=0.5x^2-3\). Felix’s sequence produces \(g_F(x)=0.5x^2-1.5\).
Quadratic forms and transformations 40
Let \(f(x)=x^2\). The graph of \(g\) is produced by first vertically stretching the graph of \(f\) by a factor of \(3\), then shifting it down \(2\) units. The graph of \(h\) is produced by performing the same transformations in the opposite order. Show algebraically that the vertical distance between the graphs of \(g\) and \(h\) is constant, and state that distance.

Hints

- Write a function rule after each transformation in each sequence. - When the stretch is second, it applies to the entire shifted output. - A constant vertical distance means the difference of the two functions is constant. - Simplify \(g(x)-h(x)\).

Solution

1. Stretch first, then shift: \(g(x)=3f(x)-2=3x^2-2\). 2. Shift first, then stretch the entire output: \(h(x)=3(f(x)-2)=3(x^2-2)=3x^2-6\). 3. Compute the difference: \(g(x)-h(x)=(3x^2-2)-(3x^2-6)=4\). 4. Since the difference is \(4\) for every real \(x\), the vertical distance between the graphs is constantly \(4\) units.

Answer

The vertical distance is \(4\) units because \(g(x)=3x^2-2\), \(h(x)=3x^2-6\), and \(g(x)-h(x)=4\) for every real \(x\).
Quadratic forms and transformations 41
Let \(f(x)=x^2-1\). Apply each transformation sequence. Sequence A: Reflect across the x-axis, then shift up \(4\) units to obtain \(g\). Sequence B: Shift up \(4\) units, then reflect across the x-axis to obtain \(h\). Find the vertex of each resulting graph and use the vertices to explain why the order of the transformations matters.

Hints

- A reflection across the x-axis changes the sign of the entire output. - Track the vertex after each transformation. - When a reflection is performed after a shift, the shifted output is also reflected. - One differing characteristic point is enough to show that the graphs are different.

Solution

1. The vertex of \(f(x)=x^2-1\) is \((0, -1)\). 2. Sequence A gives \(-f(x)=-(x^2-1)=-x^2+1\), then \(g(x)=-x^2+5\). Its vertex is \((0, 5)\). 3. Sequence B gives \(f(x)+4=x^2+3\), then \(h(x)=-(x^2+3)=-x^2-3\). Its vertex is \((0, -3)\). 4. Since the vertices differ, the resulting functions and graphs differ.

Answer

The vertex of \(g\) is \((0, 5)\), and the vertex of \(h\) is \((0, -3)\). The different vertices show that changing the order changes the result.
Quadratic forms and transformations 42
Let \(f(x)=x^2\). Consider two transformations: reflecting the graph across the x-axis and shifting it vertically by \(c\) units. Find the value of \(c\) for which the final graph is the same regardless of the order of the transformations. Justify your answer algebraically.

Hints

- Write a function rule for each order. - A reflection across the x-axis multiplies the entire current output by \(-1\). - Identical functions must have equal outputs for every input. - Set the two final expressions equal and solve for \(c\).

Solution

1. Reflect first, then shift: \(g(x)=-f(x)+c=-x^2+c\). 2. Shift first, then reflect the entire output: \(h(x)=-(f(x)+c)=-x^2-c\). 3. For the results to be identical, set \(-x^2+c=-x^2-c\). 4. This gives \(c=-c\), so \(2c=0\) and \(c=0\). 5. When \(c=0\), both sequences produce \(-x^2\).

Answer

\(c=0\). Only a zero vertical shift makes the two transformation orders produce the same function.
Quadratic forms and transformations 43
The parent function \(f(x)=x^2\) has a global minimum at \((0, 0)\). The function \(g(x)=-2(x-3)^2+5\) is obtained by transforming \(f\). 1. Describe the transformations. 2. State the coordinates of the transformed vertex. 3. Explain why the transformed vertex is a maximum rather than a minimum.

Hints

- Read the transformations from the vertex-form parameters. - Track the original vertex through the horizontal and vertical shifts. - Determine how a negative outside coefficient changes the opening direction.

Solution

1. Shift the graph right \(3\) units, vertically stretch by a factor of \(2\), reflect across the x-axis, and shift up \(5\) units. 2. The vertex moves from \((0, 0)\) to \((3, 5)\). 3. The negative outside coefficient reflects the upward-opening parabola across the x-axis, so the transformed parabola opens downward and its vertex is a maximum.

Answer

1. Shift right \(3\) units, vertically stretch by a factor of \(2\), reflect across the x-axis, and shift up \(5\) units. 2. \((3, 5)\) 3. The reflection across the x-axis changes the minimum into a maximum.
Quadratic forms and transformations 44
Let \(f(x)=x^2-16\). Its graph is symmetric about the y-axis and has zeros at \(x=-4\) and \(x=4\). a) Describe the symmetry and find the zeros of \(p(x)=f(x-2)\). b) Describe the symmetry and find the zeros of \(q(x)=f(0.5x)\). c) Explain how multiplying every output by a nonzero constant, \(g(x)=c\cdot f(x)\), affects the zeros of \(f\).

Hints

- A horizontal shift moves the symmetry line and the zeros by the same amount. - A factor inside the input changes horizontal scale. - Determine when a product \(c\cdot f(x)\) can equal zero. - Points on the x-axis have output \(0\).

Solution

1. The rule \(p(x)=f(x-2)\) shifts the graph right \(2\) units. Its symmetry line becomes \(x=2\), and its zeros move to \(-4+2=-2\) and \(4+2=6\). 2. The rule \(q(x)=f(0.5x)\) horizontally stretches the graph by a factor of \(2\). It remains symmetric about the y-axis, and its zeros become \(-8\) and \(8\). 3. If \(c\ne 0\), then \(c\cdot f(x)=0\) exactly when \(f(x)=0\). Therefore, a nonzero vertical scale factor does not change the zeros.

Answer

a) Symmetric about \(x=2\); zeros at \(x=-2\) and \(x=6\) b) Symmetric about the y-axis; zeros at \(x=-8\) and \(x=8\) c) A nonzero vertical scale factor does not change the zeros.
Quadratic forms and transformations 45
Consider the family of parabolas \(f_k(x)=-\frac{2}{k}x^2+8x+1\), where \(k>0\). Find an equation for the locus of all vertices.

Hints

- Rewrite the quadratic in vertex form. - Express both vertex coordinates in terms of \(k\). - Eliminate \(k\) and include the restriction implied by \(k>0\).

Solution

1. Rewrite the function in vertex form: \(f_k(x)=-\frac{2}{k}(x-2k)^2+8k+1\). 2. Thus, the vertex has coordinates \((2k,8k+1)\). 3. From \(x=2k\), obtain \(k=\frac{x}{2}\). Substitute into the y-coordinate: \(y=8\left(\frac{x}{2}\right)+1=4x+1\). 4. Because \(k>0\), the vertex satisfies \(x>0\). Therefore, the locus is \(y=4x+1\), for \(x>0\).

Answer

\(y=4x+1\), for \(x>0\)
Quadratic forms and transformations 46
Let \(f(x)=x^2-3x\). Write rules for the transformed functions. a) The graph of \(g\) is obtained by reflecting the graph of \(f\) across the x-axis and then shifting it up \(2\) units. b) The graph of \(h\) is obtained by shifting the graph of \(f\) right \(4\) units and then vertically compressing it by a factor of \(0.5\).

Hints

- A reflection across the x-axis changes the sign of the entire function. - A horizontal shift changes the input. - Vertical shifts and scales act on the output. - Use parentheses around the entire expression before applying an outside scale factor.

Solution

1. Reflecting across the x-axis gives \(-f(x)=-(x^2-3x)=-x^2+3x\). 2. Shifting up \(2\) units gives \(g(x)=-x^2+3x+2\). 3. Shifting right \(4\) units gives \(f(x-4)=(x-4)^2-3(x-4)\). 4. Vertically compressing the entire output gives \(h(x)=0.5[(x-4)^2-3(x-4)]\).

Answer

a) \(g(x)=-x^2+3x+2\) b) \(h(x)=0.5[(x-4)^2-3(x-4)]\)
Quadratic forms and transformations 47
Let \(p(x)=x^2-5x\). Describe how each graph is obtained from the graph of \(p\). a) \(q(x)=(x+3)^2-5(x+3)\) b) \(r(x)=-2x^2+10x-3\)

Hints

- Look for a complete replacement of \(x\) by \(x+3\). - Rewrite each rule so that \(p(x)\) appears as a single expression. - An outside factor changes vertical scale and may create a reflection. - A constant added outside the function creates a vertical shift.

Solution

1. Since \(q(x)=p(x+3)\), the graph of \(q\) is the graph of \(p\) shifted left \(3\) units. 2. Factor the first two terms of \(r\): \(r(x)=-2(x^2-5x)-3=-2p(x)-3\). 3. Therefore, the graph of \(r\) is obtained by vertically stretching the graph of \(p\) by a factor of \(2\), reflecting it across the x-axis, and shifting it down \(3\) units.

Answer

a) Shift left \(3\) units. b) Vertically stretch by a factor of \(2\), reflect across the x-axis, and shift down \(3\) units.
Quadratic forms and transformations 48
A quadratic function has the form \(h(x)=ax^2\), and its graph passes through \(P(2,4)\). a) Find \(a\). b) Evaluate \(h(-3)\). c) Another function, \(k(x)=bx^2\), passes through \(Q(1,-2)\). Compare its graph with the graph of \(h\): identify the quadrants containing the graph of \(k\), and describe a reflection and vertical stretch that map the graph of \(h\) onto the graph of \(k\).

Hints

- Use the given point to solve for the unknown coefficient. - Evaluate the square before multiplying by the coefficient. - Compare \(k(x)\) directly with \(h(x)\) by writing \(k(x)=ch(x)\). - A negative output scaling reflects a graph across the x-axis.

Solution

1. Substitute \((2,4)\): \(4=a(2^2)=4a\), so \(a=1\). 2. Then \(h(-3)=(-3)^2=9\). 3. Substitute \((1,-2)\) into \(k(x)=bx^2\): \(-2=b(1)^2\), so \(b=-2\). 4. Since \(k(x)=-2x^2=-2h(x)\), reflect the graph of \(h\) across the x-axis and stretch it vertically by a factor of \(2\). Except for the origin, the graph of \(k\) lies in Quadrants III and IV.

Answer

a) \(a=1\) b) \(h(-3)=9\) c) \(k(x)=-2h(x)\). Reflect the graph of \(h\) across the x-axis and stretch it vertically by a factor of \(2\). Except for the origin, the graph lies in Quadrants III and IV.
Quadratic forms and transformations 49
In \(x^2 + px + 12 = 0\), one real zero is three times the other. Find all possible values of \(p\).

Hints

- Represent the zeros as \(r\) and \(3r\). - Use the zeros to write the monic quadratic as a product of two linear factors. - Expand the factors and compare coefficients with the given quadratic.

Solution

1. Let the zeros be \(r\) and \(3r\). 2. Because the quadratic is monic, its factored form is \((x-r)(x-3r)\). 3. Expand: \((x-r)(x-3r)=x^2-4rx+3r^2\). 4. Compare the constant term with \(x^2+px+12\): \(3r^2=12\), so \(r^2=4\) and \(r=2\) or \(r=-2\). 5. Compare the coefficient of \(x\): \(p=-4r\). Thus \(p=-8\) when \(r=2\), and \(p=8\) when \(r=-2\).

Answer

\(p = -8\) or \(p = 8\)
Quadratic forms and transformations 50
The vertex of a downward-opening parabola with leading coefficient \(-1\) moves along the graph of \(h(x)=\frac{1}{x}\) for \(x>0\). a) Let \(a\) be the x-coordinate of the vertex. Write the family \(g_a\) in vertex form. b) Suppose the vertex also lies on the line \(y=x-\frac{3}{2}\). Find \(a\) and write the corresponding member of the family.

Hints

- Use the graph of \(h\) to write the vertex coordinates in terms of \(a\). - Use the vertex coordinates to write the parabola in vertex form. - For part b, the vertex's y-coordinate must satisfy both equations that describe its location. - Use the condition \(a>0\) when choosing among algebraic solutions.

Solution

1. Because the vertex lies on \(h(x)=\frac{1}{x}\), its coordinates are \(\left(a,\frac{1}{a}\right)\), where \(a>0\). 2. A downward-opening parabola with leading coefficient \(-1\) and that vertex is \(g_a(x)=-(x-a)^2+\frac{1}{a}\). 3. If the vertex also lies on \(y=x-\frac{3}{2}\), then \(\frac{1}{a}=a-\frac{3}{2}\). 4. Multiply by \(2a\): \(2=2a^2-3a\), so \(2a^2-3a-2=0=(a-2)(2a+1)\). 5. The candidates are \(a=2\) and \(a=-\frac{1}{2}\). Since \(a>0\), \(a=2\). 6. Therefore, the corresponding parabola is \(g_2(x)=-(x-2)^2+\frac{1}{2}\).

Answer

a) \(g_a(x)=-(x-a)^2+\frac{1}{a}\), where \(a>0\) b) \(a=2\), so \(g_2(x)=-(x-2)^2+\frac{1}{2}\)
Quadratic forms and transformations 51
The table shows values of a quadratic function \(f\). <table> <tr><td>\(x\)</td><td>\(-2\)</td><td>\(-1\)</td><td>\(0\)</td><td>\(1\)</td><td>\(2\)</td><td>\(3\)</td></tr> <tr><td>\(f(x)\)</td><td>\(-4\)</td><td>\(2\)</td><td>\(4\)</td><td>\(2\)</td><td>\(-4\)</td><td>\(-14\)</td></tr> </table> a) Create value tables for \(g(x)=2.5f(x)\) and \(h(x)=-0.5f(x)\). b) Describe how the graphs of \(g\) and \(h\) are obtained from the graph of \(f\). c) Find a possible rule for \(f\), then write rules for \(g\) and \(h\).

Hints

- Multiply each given output by the outside scale factor. - A negative scale factor also reflects the graph across the x-axis. - Use finite differences to identify the function type. - An outside factor distributes to every term of the function rule.

Solution

1. Multiply each output by \(2.5\): the values of \(g\) are \(-10, 5, 10, 5, -10, -35\). 2. Multiply each output by \(-0.5\): the values of \(h\) are \(2, -1, -2, -1, 2, 7\). 3. The graph of \(g\) is a vertical stretch of the graph of \(f\) by a factor of \(2.5\). The graph of \(h\) is a vertical compression by a factor of \(0.5\) followed by a reflection across the x-axis. 4. The second differences of the \(f(x)\)-values are constant at \(-4\), so let \(f(x)=ax^2+bx+c\). From \(f(0)=4\), \(c=4\). Since \(f(-1)=f(1)\), \(b=0\). From \(f(1)=2\), \(a+4=2\), so \(a=-2\). 5. Therefore, \(f(x)=-2x^2+4\), \(g(x)=2.5(-2x^2+4)=-5x^2+10\), and \(h(x)=-0.5(-2x^2+4)=x^2-2\).

Answer

a) \(g(x)\)-values: \(-10, 5, 10, 5, -10, -35\) \(h(x)\)-values: \(2, -1, -2, -1, 2, 7\) b) \(g\) is a vertical stretch by a factor of \(2.5\). \(h\) is a vertical compression by a factor of \(0.5\) and a reflection across the x-axis. c) \(f(x)=-2x^2+4\), \(g(x)=-5x^2+10\), and \(h(x)=x^2-2\)
Quadratic forms and transformations 52
Let \(p(x)=x^2-4\) and \(q(x)=k\cdot p(x)\). The table gives several values of \(q\). <table> <tr><td>\(x\)</td><td>\(-2\)</td><td>\(-1\)</td><td>\(0\)</td><td>\(1\)</td><td>\(2\)</td></tr> <tr><td>\(q(x)\)</td><td>\(0\)</td><td>\(6\)</td><td>\(8\)</td><td>\(6\)</td><td>\(0\)</td></tr> </table> a) Find \(k\). b) Describe the transformation from the graph of \(p\) to the graph of \(q\). c) Find the zeros of \(p\) and compare them with the zeros of \(q\). State a general conclusion for transformations of the form \(k\cdot f(x)\), where \(k\ne 0\).

Hints

- Use one table value and the corresponding value of \(p\) to solve for \(k\). - The sign and magnitude of an outside factor determine reflection and vertical scale. - A product is zero when one of its factors is zero. - Consider what happens to an output that is already \(0\).

Solution

1. Use \(x=0\): \(p(0)=-4\) and \(q(0)=8\), so \(8=k(-4)\). Therefore, \(k=-2\). 2. Multiplying by \(-2\) vertically stretches the graph by a factor of \(2\) and reflects it across the x-axis. 3. Solve \(p(x)=0\): \(x^2-4=0\), so \(x=-2\) or \(x=2\). 4. Since \(q(x)=-2p(x)\) and \(-2\ne0\), \(q(x)=0\) exactly when \(p(x)=0\). Thus, both functions have the same zeros. 5. In general, multiplying a function by a nonzero constant does not change the x-values where the function equals \(0\).

Answer

a) \(k=-2\) b) Vertically stretch by a factor of \(2\) and reflect across the x-axis. c) Both functions have zeros at \(x=-2\) and \(x=2\). Multiplying a function by a nonzero constant does not change its zeros.
Quadratic forms and transformations 53
Consider \(h(x)=(0.2x-4)^2\). Is \(h\) a polynomial function? Describe one sequence of transformations that produces the graph of \(h\) from the graph of the parent function \(f(x)=x^2\).

Hints

- The power of a linear expression expands to a polynomial. - Factor the coefficient of \(x\) from the entire input. - An input coefficient of \(0.2\) creates a horizontal stretch by its reciprocal. - Use the factored form to read the horizontal shift.

Solution

1. The expression \((0.2x-4)^2\) is the square of a linear polynomial, so its expansion is a polynomial of degree \(2\). 2. Factor the input: \(0.2x-4=0.2(x-20)\). Thus, \(h(x)=f(0.2(x-20))\). 3. Horizontally stretch the graph of \(f\) by a factor of \(5\). 4. Shift the result right \(20\) units.

Answer

Yes. \(h\) is a quadratic polynomial function. One sequence is a horizontal stretch by a factor of \(5\), followed by a shift right \(20\) units.
Quadratic forms and transformations 54
Consider the family of parabolas \(p_k(x)=x^2-2kx+3k\), where \(k\) is real. a) Find the equation of the locus containing all vertices of the family. b) For which values of \(k\) is the vertex above the line \(y=2\)?

Hints

- Write the vertex coordinates in terms of \(k\). - Eliminate \(k\) between the x- and y-coordinates. - “Above \(y=2\)” means the vertex's y-coordinate is greater than \(2\). - Factor the resulting quadratic inequality.

Solution

1. The x-coordinate of the vertex is \(x_v=-\frac{-2k}{2}=k\). 2. Substitute \(x=k\): \(y_v=k^2-2k^2+3k=-k^2+3k\). Thus, the vertex is \((k, -k^2+3k)\). 3. Since \(x=k\), eliminate the parameter to obtain the locus \(y=-x^2+3x\). 4. The vertex is above \(y=2\) when \(-k^2+3k>2\). This is equivalent to \((k-1)(k-2)<0\), which holds for \(1<k<2\).

Answer

a) \(y=-x^2+3x\) b) \(1<k<2\)
Quadratic forms and transformations 55
For each \(a>0\), define \(g_a(x)=\sqrt{a}\,x-\frac{1}{2}x^2\). a) Show that every graph in the family passes through the origin. b) Find the coordinates of the maximum point \(H_a\) in terms of \(a\). c) Find an equation for the locus containing all points \(H_a\).

Hints

- Evaluate the function at \(x=0\). - Use the vertex formula for a quadratic. - Eliminate the parameter \(a\) from the two vertex coordinates. - Use \(a>0\) to determine the relevant part of the locus.

Solution

1. Evaluate the function at zero: \(g_a(0)=0\). Therefore, every graph passes through \((0, 0)\). 2. The vertex x-coordinate is \(x_H=-\frac{\sqrt{a}}{2(-\frac{1}{2})}=\sqrt{a}\). 3. Substitute \(x=\sqrt{a}\): \(g_a(\sqrt{a})=a-\frac{1}{2}a=\frac{a}{2}\). Thus, \(H_a=\left(\sqrt{a}, \frac{a}{2}\right)\). 4. Since \(x=\sqrt{a}\), we have \(a=x^2\), with \(x>0\). Substitute into \(y=\frac{a}{2}\) to obtain \(y=\frac{1}{2}x^2\) for \(x>0\).

Answer

a) \(g_a(0)=0\), so every graph passes through \((0, 0)\). b) \(H_a=\left(\sqrt{a}, \frac{a}{2}\right)\) c) \(y=\frac{1}{2}x^2\) for \(x>0\)
Quadratic forms and transformations 56
In each panel, \(f(x)=x^2\) is the parent function. a) Describe the shifts that produce \(g\), and write its rule. b) Describe the shifts and vertical stretch that produce \(g\), and write its rule.
Figure for problem 532321

Hints

- Compare the vertices in each panel. - Use the vertex to find the horizontal and vertical shifts. - In part b, use another readable point to find the vertical scale factor.

Solution

1. In part a, the vertex moves from \((0, 0)\) to \((2, 1)\). This is a shift right \(2\) and up \(1\), so \(g(x)=(x-2)^2+1\). 2. In part b, the vertex is \((-1, -3)\), giving \(g(x)=a(x+1)^2-3\). 3. The point \((0, -1)\) gives \(-1=a-3\), so \(a=2\). 4. Thus \(g(x)=2(x+1)^2-3\): a vertical stretch by \(2\), a shift left \(1\), and a shift down \(3\).

Answer

a) Shift right \(2\) units and up \(1\) unit; \(g(x)=(x-2)^2+1\). b) Vertically stretch by a factor of \(2\), shift left \(1\) unit, and shift down \(3\) units; \(g(x)=2(x+1)^2-3\).
Quadratic forms and transformations 57
Each panel shows a transformed quadratic graph. a) In panel 1, the graph of \(g\) is obtained from \(f(x)=x^2\) by shifts, a reflection, and/or a vertical stretch or compression. Describe the transformations and write \(g\) in vertex form. b) In panel 2, the graph of \(h\) is obtained from \(f(x)=x^2\). Describe the transformations and write \(h\) in vertex form.
Figure for problem 532322

Hints

- Begin with the vertex in each panel. - Write a vertex-form rule with an unknown outside coefficient. - Use a second readable point to determine that coefficient. - The sign and magnitude of the coefficient determine reflection and vertical scale.

Solution

1. In panel 1, the vertex is \((3, 4)\), so \(g(x)=a(x-3)^2+4\). 2. The point \((1, 2)\) lies on the graph. Substitution gives \(2=4a+4\), so \(a=-0.5\). Thus, \(g(x)=-0.5(x-3)^2+4\). 3. The transformations are a vertical compression by a factor of \(0.5\), a reflection across the x-axis, a shift right \(3\) units, and a shift up \(4\) units. 4. In panel 2, the vertex is \((-2, -1)\), so \(h(x)=a(x+2)^2-1\). 5. The point \((0, 7)\) lies on the graph. Substitution gives \(7=4a-1\), so \(a=2\). Thus, \(h(x)=2(x+2)^2-1\). 6. The transformations are a vertical stretch by a factor of \(2\), a shift left \(2\) units, and a shift down \(1\) unit.

Answer

a) Vertically compress by a factor of \(0.5\), reflect across the x-axis, shift right \(3\) units, and shift up \(4\) units; \(g(x)=-0.5(x-3)^2+4\). b) Vertically stretch by a factor of \(2\), shift left \(2\) units, and shift down \(1\) unit; \(h(x)=2(x+2)^2-1\).
Quadratic forms and transformations 58
Panels A, B, C, and D show the graph of \(f(x)=0.5x^2-2\) and three translated functions: \(g(x)=f(x)-2\) \(h(x)=f(x+2)\) \(k(x)=f(x-2)\) Match each function \(f\), \(g\), \(h\), and \(k\) to its graph. Justify your choices using the vertices.
Figure for problem 532385

Hints

- Begin with the vertex of the original quadratic. - A constant outside the function creates a vertical shift. - A change inside the input creates a horizontal shift in the opposite direction. - Match the translated vertices to the panels.

Solution

1. The graph of \(f\) has vertex \((0, -2)\), which appears in panel B. 2. The graph of \(g(x)=f(x)-2\) shifts down \(2\) units, so its vertex is \((0, -4)\). This is panel C. 3. The graph of \(h(x)=f(x+2)\) shifts left \(2\) units, so its vertex is \((-2, -2)\). This is panel D. 4. The graph of \(k(x)=f(x-2)\) shifts right \(2\) units, so its vertex is \((2, -2)\). This is panel A.

Answer

\(f\): B; \(g\): C; \(h\): D; \(k\): A
Quadratic forms and transformations 59
Let \(f(x)=x^2-4x+7\). a) Verify algebraically that \(f(2+a)=f(2-a)\) for every real \(a\). b) Explain why this identity proves that the graph is symmetric about \(x=2\).

Hints

- Rewrite the quadratic using \(x-2\), or substitute both expressions directly. - Compare the even powers of \(a\). - Interpret \(2+a\) and \(2-a\) as inputs equally far from the proposed axis.

Solution

1. Rewrite the quadratic as \(f(x)=(x-2)^2+3\). 2. Substitute \(x=2+a\): \(f(2+a)=a^2+3\). 3. Substitute \(x=2-a\): \(f(2-a)=(-a)^2+3=a^2+3\). 4. Therefore, \(f(2+a)=f(2-a)\) for every real \(a\). 5. The inputs \(2+a\) and \(2-a\) are equally far from \(2\). Equal outputs at every such pair mean the graph is symmetric about \(x=2\).

Answer

a) \(f(2+a)=f(2-a)=a^2+3\) b) Points equally far to the left and right of \(x=2\) have equal y-values, so the graph is symmetric about \(x=2\).
Quadratic forms and transformations 60
The function in panel \(f\) is \(f(x)=-x^2+2\). The other panels show functions of the form \(g(x)=af(x-c)+d\). Determine \(a\), \(c\), and \(d\) for \(g_1\) and \(g_2\).
Figure for problem 532468

Hints

- Use the vertex x-coordinate to find \(c\). - Compare opening directions and widths to find \(a\). - Substitute the transformed vertex into \(af(x-c)+d\) to find \(d\); a reflection also changes the parent vertex height.

Solution

1. The graph of \(f\) has vertex \((0, 2)\) and opens downward. 2. The vertex of \(g_1\) is \((2, 1)\). It opens downward with the same width, so \(a=1\), \(c=2\), and \(d=-1\), since \(f(x-2)-1=-(x-2)^2+1\). 3. The vertex of \(g_2\) is \((-1, 0)\). It opens upward with the same width, so \(a=-1\) and \(c=-1\). Because \(-f(x+1)+d=(x+1)^2-2+d\), the vertex height \(0\) gives \(d=2\).

Answer

\(g_1\): \(a=1\), \(c=2\), \(d=-1\) \(g_2\): \(a=-1\), \(c=-1\), \(d=2\)
Quadratic forms and transformations 61
Each panel compares \(f(x)=x^2\) with a transformed function \(p\). Match panels 1, 2, and 3 to a rule. Describe each transformation. \(p_1(x)=2f(x)\) \(p_2(x)=f(x-1)-2\) \(p_3(x)=f(-x)\) \(p_4(x)=-f(x)+1\) \(p_5(x)=f(2x)\) \(p_6(x)=f(x+1)+1\)
Figure for problem 532469

Hints

- Compare the vertices and opening directions. - Changes inside the input move the graph horizontally. - Compare heights at the same x-value to distinguish vertical scaling from horizontal scaling.

Solution

1. In panel 1, the vertex moves to \((1, -2)\); this is \(p_2(x)=f(x-1)-2\), a shift right \(1\) and down \(2\). 2. In panel 2, the parabola opens downward with vertex \((0, 1)\); this is \(p_4(x)=-f(x)+1\), a reflection across the x-axis followed by a shift up \(1\). 3. In panel 3, the vertex stays fixed, but the parabola is narrower and its outputs double; this is \(p_1(x)=2f(x)\), a vertical stretch by \(2\).

Answer

1: \(p_2(x)=f(x-1)-2\), shift right \(1\) unit and down \(2\) units. 2: \(p_4(x)=-f(x)+1\), reflect across the x-axis and shift up \(1\) unit. 3: \(p_1(x)=2f(x)\), vertically stretch by a factor of \(2\).
Quadratic forms and transformations 62
In each panel, \(f(x)=x^2\) is the parent function. a) Write \(g_1\) in terms of \(f\), then expand the rule. b) Write \(g_2\) in the form \(f(bx)+k\), where \(b>0\), then expand the rule.
Figure for problem 532475

Hints

- Read the vertex and opening direction in each panel. - In part a, apply the reflection to the whole parent output. - In part b, use the width to determine the positive value of \(b\). - Expand after writing each transformation rule.

Solution

1. In part a, the vertex moves to \((-1, 2)\) and the parabola opens downward. Reflect across the x-axis, shift left \(1\), and shift up \(2\). 2. Thus \(g_1(x)=-f(x+1)+2=-(x+1)^2+2=-x^2-2x+1\). 3. In part b, the vertex is \((0, -1)\). The parabola is horizontally compressed by a factor of \(\frac{1}{2}\). Since \(b>0\), the input coefficient is \(b=2\). 4. Thus \(g_2(x)=f(2x)-1=4x^2-1\). Without the condition \(b>0\), \(b=-2\) would also give the same graph.

Answer

a) \(g_1(x)=-f(x+1)+2=-x^2-2x+1\) b) \(g_2(x)=f(2x)-1=4x^2-1\)
Quadratic forms and transformations 63
The parent function in both panels is \(f(x)=(x-1)^2\). For each panel, describe the transformations that produce \(g\) and write a rule for \(g\) in terms of \(f\).
Figure for problem 532479

Hints

- Locate the vertex of each parabola. - Compare the movement of the vertices. - Check whether the transformed graph opens up or down. - Write the input and output changes separately.

Solution

1. In panel a), the vertex moves from \((1, 0)\) to \((-2, -2)\), and the width and opening stay the same. 2. Shifting left \(3\) units and down \(2\) units gives \(g(x)=f(x+3)-2\). 3. In panel b), the vertex moves from \((1, 0)\) to \((1, 1)\) while the opening reverses. 4. Reflecting across the x-axis and then shifting up \(1\) unit gives \(g(x)=-f(x)+1\).

Answer

a) Shift left \(3\) units and down \(2\) units; \(g(x)=f(x+3)-2\) b) Reflect across the x-axis and shift up \(1\) unit; \(g(x)=-f(x)+1\)
Quadratic forms and transformations 64
The parent function is \(f(x)=(x-1)^2\). Find two different sequences of transformations that produce \(g\) from \(f\). One must use reflection across the y-axis. Give the rule for \(g\) in terms of \(f\) for each sequence.
Figure for problem 532483

Hints

- Compare the two vertices. - A reflection across the y-axis replaces every input \(x\) by \(-x\). - Check whether a horizontal shift could place the vertex in the same spot. - Simplify the two rules to check that they agree.

Solution

1. The vertex moves from \((1, 0)\) to \((-1, 1)\). 2. Reflect across the y-axis to reach \((-1, 0)\), then shift up \(1\): \(g(x)=f(-x)+1\). 3. Alternatively, shift left \(2\) and then up \(1\): \(g(x)=f(x+2)+1\). 4. Both give \((x+1)^2+1\), since \((-x-1)^2=(x+1)^2\).

Answer

1) Reflect across the y-axis, then shift up \(1\) unit: \(g(x)=f(-x)+1\) 2) Shift left \(2\) units, then shift up \(1\) unit: \(g(x)=f(x+2)+1\)
Quadratic forms and transformations 65
The parent function is \(f(x)=x^2\). Show algebraically that the graph of \(h\) can be obtained both by a vertical stretch and by a horizontal compression of \(f\). Find both scale factors.
Figure for problem 534044

Hints

- Use the marked point to find the coefficient in \(h(x)=ax^2\). - Compare \(h(x)\) with a multiple of \(f(x)\). - What happens to \(x^2\) when the input \(x\) is multiplied by a number?

Solution

1. The marked point \((1, 4)\) is on \(h\). Writing \(h(x)=ax^2\) gives \(4=a\), so \(h(x)=4x^2\). 2. Since \(h(x)=4f(x)\), the vertical stretch factor is \(4\). 3. Also \(h(x)=f(2x)=(2x)^2=4x^2\). This is a horizontal compression by a factor of \(\frac{1}{2}\).

Answer

\(h(x)=4f(x)=f(2x)=4x^2\). Vertical stretch factor: \(4\). Horizontal compression factor: \(\frac{1}{2}\).
Quadratic forms and transformations 66
The parent function is \(f(x)=x^2-1\). Match \(p\) and \(q\) to curves \(u\) and \(v\). Justify each match using the vertex and one other feature. \(p(x)=f(x-1)+2\) \(q(x)=2f(x)-1\)
Figure for problem 534062

Hints

- Find the parent vertex first. - A translation does not change the width. - A vertical stretch changes the widths and the vertex height. - Check one simple input after comparing the vertices.

Solution

1. The vertex of \(f\) is \((0, -1)\). 2. The graph of \(p\) shifts right \(1\) and up \(2\) to vertex \((1, 1)\), keeping the same width. It matches curve \(u\). 3. The graph of \(q\) stretches by \(2\), then shifts down \(1\), moving its vertex to \((0, -3)\). 4. Since \(f(1)=0\), we have \(q(1)=-1\), another point on curve \(v\).

Answer

Curve \(u\) is \(p\): vertex \((1, 1)\), with the same width as \(f\). Curve \(v\) is \(q\): vertex \((0, -3)\), narrower than \(f\) because of the vertical stretch.
Quadratic forms and transformations 67
The parent function is \(f(x)=x(x-2)\). The transformed function has the form \(g(x)=af(x+c)\). Use the graph to find \(a\) and \(c\).
Figure for problem 534066

Hints

- Compare the pairs of x-intercepts. - Track the direction of the horizontal shift. - Compare corresponding vertex heights to determine the scale and reflection.

Solution

1. The zeros of \(f\) are \(0\) and \(2\), while the zeros of \(g\) are \(-1\) and \(1\). 2. Both zeros move left \(1\), so the input is \(x+1\), giving \(c=1\). 3. The parent vertex \((1,-1)\) would move to \((0,-1)\). The new vertex is \((0,1)\), so outputs are multiplied by \(-1\), giving \(a=-1\). 4. Therefore, \(g(x)=-f(x+1)=-x^2+1\).

Answer

\(a=-1\) and \(c=1\)
Quadratic forms and transformations 68
The parent graph is \(f(x)=x^2+4\). The transformed rule has the form \(g(x)=af(x-c)\), with no separate vertical shift. Find \(a\) and \(c\) from the graph, then write \(g(x)\).
Figure for problem 534068

Hints

- Compare the x-coordinates of the vertices to find \(c\). - The new opening direction tells you the sign of \(a\). - Use the vertex y-values to find \(a\); there is no extra vertical shift in the given form.

Solution

1. The vertex moves from \((0,4)\) to \((1,-2)\), so the horizontal shift gives \(c=1\). 2. The graph now opens downward, so \(a<0\). It is wider than \(f\), indicating a compression. 3. At the vertex, \(g(1)=a f(0)=4a=-2\), so \(a=-\frac{1}{2}\). 4. Therefore, \(g(x)=-\frac{1}{2}f(x-1)=-\frac{1}{2}((x-1)^2+4)\).

Answer

\(a=-\frac{1}{2}\), \(c=1\), and \(g(x)=-\frac{1}{2}((x-1)^2+4)\)
Quadratic forms and transformations 69
In each panel, the red graph is a transformation of the blue graph \(f(x)=4-x^2\). Describe the transformations and write the red function in terms of \(f\). a) Graph \(g\) b) Graph \(h\)
Figure for problem 534107

Hints

- Compare the vertices first. - Decide whether each red graph has the same width as the blue graph. - Horizontal shifts affect the input. - Vertical scaling and shifting affect the output.

Solution

1. In panel a), the vertex moves from \((0, 4)\) to \((-4, 3)\), and the width and opening direction remain unchanged. 2. Therefore, the graph shifts left \(4\) units and down \(1\) unit, so \(g(x)=f(x+4)-1\). 3. In panel b), the vertex moves from \((0, 4)\) to \((0, 3)\), and the red parabola is wider. 4. Multiplying outputs by \(0.5\) changes the vertex value from \(4\) to \(2\), and then shifting up \(1\) unit gives \(3\). Therefore, \(h(x)=0.5f(x)+1\).

Answer

a) Shift left \(4\) units and down \(1\) unit; \(g(x)=f(x+4)-1\) b) Vertically compress by a factor of \(0.5\) and shift up \(1\) unit; \(h(x)=0.5f(x)+1\)
Quadratic forms and transformations 70
A quadratic function \(g\) has vertex \(S=(2,3)\) and passes through \(A=(0,-1)\). a) Write \(g\) in vertex form. b) State the axis of symmetry and the maximum value of \(g\). c) Find the exact x-intercepts of \(g\).

Hints

- How does the vertex determine the two shifts in the form \(a(x-h)^2+k\)? - Use the second point to determine the remaining coefficient. - The vertex tells you both the symmetry line and whether the extreme value is a maximum or minimum. - For x-intercepts, set the function value equal to \(0\) and solve exactly.

Solution

1. Use vertex form \(g(x)=a(x-2)^2+3\). 2. Since \(A=(0,-1)\) lies on the graph, \(-1=a(0-2)^2+3=4a+3\). Therefore, \(a=-1\), so \(g(x)=-(x-2)^2+3\). 3. The vertex is \((2,3)\), so the axis of symmetry is \(x=2\) and the maximum value is \(3\). 4. For the x-intercepts, set \(g(x)=0\): \(0=-(x-2)^2+3\), so \((x-2)^2=3\). 5. Thus, \(x=2-\sqrt{3}\) or \(x=2+\sqrt{3}\).

Answer

a) \(g(x)=-(x-2)^2+3\) b) Axis of symmetry: \(x=2\); maximum value: \(3\) c) x-intercepts: \((2-\sqrt{3},0)\) and \((2+\sqrt{3},0)\)
Quadratic forms and transformations 71
Several quadratic functions \(f_a\), with \(a\ne0\), pass through the marked points \(P\) and \(Q\). a) Read the coordinates of \(P\) and \(Q\). Write \(f_a(x)=ax^2+bx+c\), expressing \(b\) and \(c\) in terms of \(a\). b) One member passes through \(R(2,5)\). Find \(a\). c) Give the common axis of symmetry. What happens to the vertex height as \(a\) becomes a larger and larger positive number?
Figure for problem 534364

Hints

- Read the two marked points and substitute their coordinates. - Use each common point to find how \(b\) and \(c\) depend on \(a\). - For part b, use the extra point only after writing the family rule. - The axis lies halfway between two x-coordinates with equal y-values. - Write the vertex height as an expression in \(a\) and see what happens when positive \(a\) grows.

Solution

1. The marked points are \(P=(0,3)\) and \(Q=(4,3)\). 2. Substituting \(P\) gives \(c=3\); substituting \(Q\) gives \(3=16a+4b+3\), so \(b=-4a\). 3. Hence \(f_a(x)=ax^2-4ax+3\). 4. Substituting \(R(2,5)\) gives \(5=4a-8a+3\), so \(a=-\frac{1}{2}\). 5. The equal-height points are symmetric about \(x=2\); the vertex height is \(f_a(2)=3-4a\). 6. As positive \(a\) increases, \(3-4a\) decreases without bound, so the vertex moves farther and farther down.

Answer

a) \(P=(0,3)\), \(Q=(4,3)\), and \(f_a(x)=ax^2-4ax+3\) b) \(a=-\frac{1}{2}\) c) The axis is \(x=2\); the vertex height decreases without bound as positive \(a\) increases.
Quadratic forms and transformations 72
A quadratic has vertex \((2,-9)\) and one zero at \(x=5\). a) Find the other zero. b) Write the quadratic in factored form. c) Write the same quadratic in vertex form and standard form.

Hints

- Use the axis of symmetry through the vertex to relate the two zeros. - After locating both zeros, include an unknown scale factor in factored form. - Use the vertex value to determine that scale factor. - Check that all three forms describe the same quadratic.

Solution

1. The axis of symmetry passes through the vertex, so it is \(x=2\). The zero at \(x=5\) is \(3\) units to the right of the axis, so the other zero is \(3\) units to the left: \(x=-1\). 2. Write \(f(x)=a(x+1)(x-5)\). At the vertex, \(f(2)=-9\), so \(-9=a(3)(-3)=-9a\). Thus, \(a=1\). 3. Factored form is \(f(x)=(x+1)(x-5)\). 4. Using the vertex gives \(f(x)=(x-2)^2-9\). 5. Expanding either form gives \(f(x)=x^2-4x-5\).

Answer

a) \(x=-1\) b) \(f(x)=(x+1)(x-5)\) c) Vertex form: \(f(x)=(x-2)^2-9\) Standard form: \(f(x)=x^2-4x-5\)
Quadratic forms and transformations 73
The graph shows a quadratic function \(f\). a) Use features of the graph to write \(f\) in factored form. b) Rewrite the same function in vertex form. c) Which of those two forms makes it easiest to determine where \(f(x)<0\)? Explain.
Figure for problem 555714

Hints

- Start with the x-intercepts, but remember that they do not determine the vertical scale by themselves. - Use one additional point from the graph to determine the scale factor. - The axis of symmetry lies halfway between the two zeros. - For the inequality, think about which form exposes the boundary values most directly.

Solution

1. The graph crosses the x-axis at \(x=-2\) and \(x=4\), so \(f(x)=a(x+2)(x-4)\). 2. The graph also shows the y-intercept \((0,-8)\). Substituting \(x=0\) gives \(-8=a(2)(-4)=-8a\), so \(a=1\). 3. Thus, \(f(x)=(x+2)(x-4)\). 4. The axis of symmetry is halfway between the zeros at \(x=1\), and the vertex is \((1,-9)\), so \(f(x)=(x-1)^2-9\). 5. Factored form makes the boundary zeros visible immediately, so together with the upward opening it shows \(f(x)<0\) for \(-2<x<4\).

Answer

a) \(f(x)=(x+2)(x-4)\) b) \(f(x)=(x-1)^2-9\) c) Factored form; \(f(x)<0\) for \(-2<x<4\) because the zeros are visible directly.
Quadratic forms and transformations 74
A quadratic has vertex \((1,-9)\) and y-intercept \((0,-5)\). a) Write the rule in standard form. b) Find its zeros.

Hints

- Start in the form that shows the given vertex directly. - The y-intercept can determine the remaining coefficient. - Which form is easiest to use when setting the function equal to zero?

Solution

1. The vertex gives \(f(x)=a(x-1)^2-9\). 2. Substitute the y-intercept: \(-5=a-9\), so \(a=4\). 3. Expand \(4(x-1)^2-9\) to get \(f(x)=4x^2-8x-5\). 4. Solve \(4(x-1)^2-9=0\): \((x-1)^2=\frac{9}{4}\), so \(x=1\pm\frac{3}{2}\). 5. The zeros are \(-\frac{1}{2}\) and \(\frac{5}{2}\).

Answer

a) \(f(x)=4x^2-8x-5\) b) \(x=-\frac{1}{2}\) and \(x=\frac{5}{2}\)
Quadratic forms and transformations 75
The function is \(f(x)=2x^2+kx+8\). a) Find all real values of \(k\) for which \(f\) can be written as \(f(x)=2(x-x_1)(x-x_2)\) with real zeros, including a repeated zero. b) Santiago claims, “When \(k\) is a very large positive number, the zeros are nearly symmetric about the origin.” Evaluate the claim by examining the x-coordinate of the vertex as a function of \(k\). c) Prove that if \(g(x)=ax^2+bx+c\) has two zeros that are opposites, so \(x_1=-x_2\), then \(b=0\).

Hints

- Connect the coefficients of the quadratic with the condition required for its zeros to be real. - The vertex lies halfway between the zeros. - Expand \(a(x-x_1)(x-x_2)\) and compare the coefficient of \(x\).

Solution

1. Real zeros exist when the discriminant is nonnegative: \(D=k^2-4\cdot2\cdot8=k^2-64\ge0\). 2. Solving \(k^2\ge64\) gives \(k\le-8\) or \(k\ge8\). 3. The x-coordinate of the vertex is \(x=-\frac{k}{2\cdot2}=-\frac{k}{4}\). As a positive \(k\) becomes large, the midpoint of the zeros moves farther left, not closer to the origin. The claim is false. 4. If the zeros are \(x_1\) and \(x_2\), then \(g(x)=a(x-x_1)(x-x_2)\). 5. Expanding gives \(g(x)=ax^2-a(x_1+x_2)x+ax_1x_2\). Therefore, \(b=-a(x_1+x_2)\). 6. If \(x_1=-x_2\), then \(x_1+x_2=0\), so \(b=0\).

Answer

a) \(k\le-8\) or \(k\ge8\) b) The claim is false because the midpoint of the zeros is \(-\frac{k}{4}\), which moves away from the origin as positive \(k\) increases. c) Opposite zeros have sum \(0\), and coefficient comparison gives \(b=-a(x_1+x_2)=0\).
Quadratic forms and transformations 76
Consider the family of quadratic functions \(g_k(x)=x^2-kx+2k-3\), where \(k\in\mathbb{R}\). a) Rewrite \(g_k\) in vertex form and give the vertex as a function of \(k\). b) Determine the number of real zeros of \(g_k\) for each value of \(k\). c) Show that the point \((2,1)\) lies on every graph in the family.

Hints

- Complete the square to identify the vertex. - Compare the vertex's y-coordinate with zero. - Substitute the proposed common point into the family equation.

Solution

1. Complete the square: \(g_k(x)=\left(x-\frac{k}{2}\right)^2-\frac{k^2}{4}+2k-3\). Therefore, the vertex is \(\left(\frac{k}{2},-\frac{k^2}{4}+2k-3\right)\). 2. Because each parabola opens upward, the number of real zeros depends on the vertex's y-coordinate. Factor it: \(-\frac{k^2}{4}+2k-3=-\frac{1}{4}(k-2)(k-6)\). 3. The vertex is below the x-axis when \(k<2\) or \(k>6\), on the x-axis when \(k=2\) or \(k=6\), and above it when \(2<k<6\). Therefore, the graph has two, one, or no real zeros respectively. 4. Substitute \(x=2\): \(g_k(2)=4-2k+2k-3=1\). Thus, \((2,1)\) lies on every graph.

Answer

a) \(g_k(x)=\left(x-\frac{k}{2}\right)^2-\frac{k^2}{4}+2k-3\); vertex \(\left(\frac{k}{2},-\frac{k^2}{4}+2k-3\right)\) b) Two real zeros for \(k<2\) or \(k>6\); one real zero for \(k=2\) or \(k=6\); no real zeros for \(2<k<6\) c) \(g_k(2)=1\) for every \(k\).
Quadratic forms and transformations 77
Consider the family of quadratic functions \(f_a(x)=x^2-2ax+3a\), where \(a\in\mathbb{R}\). a) Rewrite \(f_a\) in vertex form and give the vertex in terms of \(a\). b) Find an equation for the locus of all vertices. c) Determine the values of \(a\) for which \(f_a\) has two, one, or no real zeros. d) Show that every graph in the family passes through one common point, and give its coordinates.

Hints

- Complete the square to find the vertex. - Eliminate the parameter from the vertex coordinates. - Compare the vertex's y-coordinate with zero to count real zeros. - Collect the terms containing \(a\) to find where the function value is independent of the parameter.

Solution

1. Complete the square: \(f_a(x)=(x-a)^2-a^2+3a\). Therefore, the vertex is \((a,-a^2+3a)\). 2. At a vertex, \(x=a\). Replacing \(a\) with \(x\) in the y-coordinate gives the locus \(y=-x^2+3x\). 3. Each parabola opens upward, so the number of real zeros depends on \(-a^2+3a=a(3-a)\). It is negative for \(a<0\) or \(a>3\), zero for \(a=0\) or \(a=3\), and positive for \(0<a<3\). 4. Thus, there are two real zeros for \(a<0\) or \(a>3\), one real zero for \(a=0\) or \(a=3\), and no real zeros for \(0<a<3\). 5. Rewrite \(f_a(x)=x^2+a(3-2x)\). For the value to be independent of \(a\), require \(3-2x=0\), so \(x=\frac{3}{2}\). Then \(y=\frac{9}{4}\). The common point is \(\left(\frac{3}{2},\frac{9}{4}\right)\).

Answer

a) \(f_a(x)=(x-a)^2-a^2+3a\); vertex \((a,-a^2+3a)\) b) \(y=-x^2+3x\) c) Two real zeros for \(a<0\) or \(a>3\); one for \(a=0\) or \(a=3\); none for \(0<a<3\) d) \(\left(\frac{3}{2},\frac{9}{4}\right)\)
Quadratic forms and transformations 78
Consider the family of quadratic functions \(h_t(x)=-x^2+tx-2t+4\), where \(t\in\mathbb{R}\). a) Rewrite \(h_t\) in vertex form and give the vertex in terms of \(t\). b) Find an equation for the locus of all vertices. c) For what value of \(t\) is the graph tangent to the x-axis? d) Show that every graph in the family passes through a common point on the x-axis.

Hints

- Complete the square to identify the vertex. - Eliminate \(t\) from the vertex coordinates. - When does the vertex of a downward-opening parabola lie on the x-axis? - Collect the terms containing \(t\) to find a parameter-independent point.

Solution

1. Complete the square: \(h_t(x)=-\left(x-\frac{t}{2}\right)^2+\frac{(t-4)^2}{4}\). 2. Therefore, the vertex is \(\left(\frac{t}{2},\frac{(t-4)^2}{4}\right)\). 3. From \(x=\frac{t}{2}\), obtain \(t=2x\). Substitution gives the vertex locus \(y=(x-2)^2\). 4. A downward-opening parabola is tangent to the x-axis when its vertex lies on the axis. Thus, \(\frac{(t-4)^2}{4}=0\), so \(t=4\). 5. Rewrite \(h_t(x)=-x^2+4+t(x-2)\). At \(x=2\), the parameter term is zero and \(h_t(2)=0\). Therefore, every graph passes through \((2,0)\).

Answer

a) \(h_t(x)=-\left(x-\frac{t}{2}\right)^2+\frac{(t-4)^2}{4}\); vertex \(\left(\frac{t}{2},\frac{(t-4)^2}{4}\right)\) b) \(y=(x-2)^2\) c) \(t=4\) d) \((2,0)\)
Quadratic forms and transformations 79
Consider the family of quadratic functions \(f_a(x)=ax^2-4x+a\), where \(a\in\mathbb{R}\setminus\{0\}\). a) Show that every vertex lies on the curve \(y=\frac{2}{x}-2x\). b) Find the values of \(a\) for which the vertex is a maximum below the x-axis.

Hints

- Use the vertex formula for a quadratic. - Express the vertex coordinates in terms of \(a\), then eliminate the parameter. - Determine when the parabola opens downward. - Account for the sign of \(a\) when solving the inequality for the vertex's height.

Solution

1. The vertex formula gives \(x_v=\frac{2}{a}\). 2. Evaluate the function at the vertex: \(y_v=f_a\left(\frac{2}{a}\right)=a-\frac{4}{a}\). 3. From \(x=\frac{2}{a}\), obtain \(a=\frac{2}{x}\). Substituting gives \(y=\frac{2}{x}-2x\), with \(x\ne0\). 4. The vertex is a maximum when the parabola opens downward, so \(a<0\). 5. It lies below the x-axis when \(a-\frac{4}{a}<0\). Since \(a<0\), multiplying by \(a\) reverses the inequality: \(a^2-4>0\). Combined with \(a<0\), this gives \(a<-2\).

Answer

a) \(y=\frac{2}{x}-2x\), for \(x\ne0\) b) \(a<-2\)
Quadratic forms and transformations 80
Let \(g_t(x)=tx^2-4x\), where \(t>0\). 1. Find the vertex \(S_t\) in terms of \(t\). 2. Find the equation of the locus containing all vertices \(S_t\). 3. Which vertex is closest to \(Q(0, -5)\)? Find the corresponding value of \(t\).

Hints

- Use the vertex formula for a quadratic. - Find a direct relation between the vertex coordinates that does not contain \(t\). - Minimize the square of the distance instead of the distance itself. - Can you rewrite the distance expression using one positive quantity that appears repeatedly?

Solution

1. The vertex x-coordinate is \(x_v=-\frac{-4}{2t}=\frac{2}{t}\). Substitution gives \(y_v=t\left(\frac{2}{t}\right)^2-4\left(\frac{2}{t}\right)=-\frac{4}{t}\). Thus, \(S_t=\left(\frac{2}{t}, -\frac{4}{t}\right)\). 2. Since \(y_v=-2x_v\) and \(t>0\) implies \(x_v>0\), the locus is \(y=-2x\) for \(x>0\). 3. The squared distance to \(Q\) is \(d^2(t)=\left(\frac{2}{t}\right)^2+\left(5-\frac{4}{t}\right)^2\). 4. Let \(z=\frac{1}{t}>0\). Then \(d^2=4z^2+(5-4z)^2=20z^2-40z+25\). This quadratic has its minimum at \(z=1\), so \(t=1\). 5. The closest vertex is \(S_1=(2, -4)\).

Answer

1. \(S_t=\left(\frac{2}{t}, -\frac{4}{t}\right)\) 2. \(y=-2x\) for \(x>0\) 3. \(t=1\); the closest vertex is \((2, -4)\).
Quadratic forms and transformations 81
The figure shows a family of parabolas whose vertices lie on the same line. Find one possible equation for the family \(f_t(x)\).
Figure for problem 534396

Hints

- Read the coordinates of the vertices. - Find a relationship between each vertex's x- and y-coordinates. - Use vertex form and a visible point to determine the vertical stretch.

Solution

1. The vertices shown are \((-2, 2)\), \((0, 0)\), \((2, -2)\), and \((4, -4)\). They follow the pattern \((t, -t)\). 2. A parabola with vertex \((t, -t)\) has the form \(f_t(x)=a(x-t)^2-t\). 3. The graph with vertex \((0, 0)\) passes through \((1, 1)\), so \(a=1\). 4. Therefore, one possible family is \(f_t(x)=(x-t)^2-t\).

Answer

\(f_t(x)=(x-t)^2-t\)

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