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Average rate of change for quadratics

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5512989
The table shows values of a quadratic function \(f\). <table><tr><th>\(x\)</th><td>\(-1\)</td><td>\(0\)</td><td>\(1\)</td><td>\(2\)</td></tr><tr><th>\(f(x)\)</th><td>\(4\)</td><td>\(1\)</td><td>\(0\)</td><td>\(1\)</td></tr></table> Find the average rate of change of \(f\) from \(x=-1\) to \(x=2\).

Hints

- Use only the function values at the two endpoints of the interval. - Compare the change in output with the change in input. - Keep the endpoint order the same in both differences.

Solution

1. From the table, \(f(-1)=4\) and \(f(2)=1\). 2. The average rate of change is \(\frac{f(2)-f(-1)}{2-(-1)}=\frac{1-4}{3}=-1\).

Answer

\(-1\)
5512999
Let \(f(x)=2x^2-3x+1\). Find the average rate of change of \(f\) on the interval \([1,4]\).

Hints

- Evaluate the quadratic at each endpoint of the interval. - Form a quotient using the change in function value over the change in input. - Check that the endpoint order is consistent in the numerator and denominator.

Solution

1. Evaluate the endpoints: \(f(1)=2-3+1=0\) and \(f(4)=2\cdot4^2-3\cdot4+1=21\). 2. The average rate of change is \(\frac{f(4)-f(1)}{4-1}=\frac{21-0}{3}=7\).

Answer

\(7\)
5513009
A ball's height is modeled by \(h(t)=-16t^2+48t+5\), where \(t\) is time in seconds and \(h(t)\) is height in feet. Find the average rate of change of the height from \(t=0.5\) to \(t=1.5\), and interpret the result in context.

Hints

- Evaluate the height at the beginning and end of the stated time interval. - Divide the change in height by the elapsed time. - Include units and describe what the sign of the rate means for the ball's height.

Solution

1. Evaluate the endpoint heights: \(h(0.5)=25\) and \(h(1.5)=41\). 2. The average rate of change is \(\frac{41-25}{1.5-0.5}=16\,\text{ft/s}\). 3. Over this interval, the ball's height increased by an average of \(16\) feet for each second of elapsed time.

Answer

\(16\,\text{ft/s}\). From \(0.5\) to \(1.5\) seconds, the ball's height increased at an average rate of \(16\) feet per second.
5513019
Let \(f(x)=x^2-4x+1\). a) Find the average rate of change of \(f\) on \([0,2]\). b) Find the average rate of change of \(f\) on \([2,4]\). c) Explain what the two rates show about the behavior of the parabola on the two intervals.

Hints

- Evaluate the function at \(0\), \(2\), and \(4\). - Compute each endpoint-to-endpoint change using the same quotient structure. - Compare both the signs and magnitudes of the two rates with the location of the vertex.

Solution

a) \(f(0)=1\) and \(f(2)=-3\), so the average rate of change is \(\frac{-3-1}{2-0}=-2\). b) \(f(2)=-3\) and \(f(4)=1\), so the average rate of change is \(\frac{1-(-3)}{4-2}=2\). c) The negative rate on \([0,2]\) shows the function decreases on average as it approaches the vertex at \(x=2\). The positive rate on \([2,4]\) shows it increases on average after the vertex. The equal magnitudes reflect the parabola's symmetry about \(x=2\).

Answer

a) \(-2\) b) \(2\) c) The function decreases toward the vertex and then increases away from it, with equal-magnitude rates on these symmetric intervals.
5513029
The graph shows a quadratic function \(f\) with points \(A\) and \(B\) marking the endpoints of an interval. a) Read the coordinates of \(A\) and \(B\). b) Find the average rate of change of \(f\) from \(A\) to \(B\). c) Write the equation of the secant line through \(A\) and \(B\), and explain how its slope is related to your answer in part b).
Figure for problem 551302

Hints

- Read each marked point as an ordered pair before calculating. - Average rate of change compares the vertical change with the horizontal change between the endpoints. - Once you know the slope, use either marked point to determine the secant line.

Solution

a) From the graph, \(A=(-1,1)\) and \(B=(2,-2)\). b) The average rate of change is \(\frac{-2-1}{2-(-1)}=\frac{-3}{3}=-1\). c) The secant line has slope \(-1\). Using point \(A\), \(y-1=-1(x+1)\), so \(y=-x\). The secant-line slope is exactly the average rate of change on the interval.

Answer

a) \(A=(-1,1)\) and \(B=(2,-2)\) b) \(-1\) c) \(y=-x\); its slope \(-1\) equals the average rate of change.
5513039
Let \(f(x)=ax^2+2x-1\). The average rate of change of \(f\) from \(x=1\) to \(x=3\) is \(10\). Find \(a\) and write the resulting quadratic function.

Hints

- Express both endpoint function values in terms of the unknown coefficient. - Use the given average rate to build an equation rather than evaluating a known function. - After solving for the coefficient, substitute it back into the original function rule.

Solution

1. Evaluate the endpoints in terms of \(a\): \(f(1)=a+1\) and \(f(3)=9a+5\). 2. Use the given average rate: \(\frac{(9a+5)-(a+1)}{3-1}=10\). 3. Simplify: \(\frac{8a+4}{2}=10\), so \(4a+2=10\) and \(a=2\). 4. Therefore, \(f(x)=2x^2+2x-1\).

Answer

\(a=2\), so \(f(x)=2x^2+2x-1\).
5513049
Let \(f(x)=2(x-3)^2-5\). For any real number \(d>0\), consider the interval \([3-d,3+d]\). a) Show without expanding the quadratic that the average rate of change of \(f\) on this interval is \(0\). b) Explain geometrically why the result is true for every \(d>0\).

Hints

- Compare how far each endpoint is from the vertex's x-coordinate. - Evaluate the two endpoint expressions in vertex form before expanding anything. - Think about what equal endpoint heights imply about a secant line.

Solution

a) The endpoints are equally far from \(x=3\). Thus, \(f(3-d)=2d^2-5\) and \(f(3+d)=2d^2-5\). Their outputs are equal, so the average rate of change is \(\frac{0}{(3+d)-(3-d)}=\frac{0}{2d}=0\). b) The parabola is symmetric about the vertical line \(x=3\). Points the same horizontal distance to the left and right of the axis have equal heights, so every secant through such a symmetric pair is horizontal.

Answer

a) \(0\) b) The interval endpoints are symmetric about \(x=3\), so they have equal function values and the secant line is horizontal.
5513059
The graph of \(f(x)=x^2-4x+1\) is shown, with the starting point \(A=(1,-2)\) marked. Find the value \(x>1\) for which the average rate of change of \(f\) from \(x=1\) to that value of \(x\) is \(3\). Then give the coordinates of the second endpoint \(B\).
Figure for problem 551305

Hints

- Write the average-rate condition using the marked starting point and an unknown second endpoint. - Simplify the resulting quotient only after noting why the unknown endpoint cannot equal the starting x-value. - After finding the x-coordinate, evaluate the quadratic to locate the second endpoint on the graph.

Solution

1. Since \(f(1)=-2\), the required condition is \(\frac{f(x)-(-2)}{x-1}=3\), with \(x>1\). 2. Substitute the function: \(\frac{x^2-4x+3}{x-1}=3\). 3. Factor the numerator: \(x^2-4x+3=(x-1)(x-3)\). Because \(x>1\), \(x\neq1\), so the quotient simplifies to \(x-3\). 4. Solve \(x-3=3\), giving \(x=6\). Then \(f(6)=36-24+1=13\), so \(B=(6,13)\).

Answer

\(x=6\), and the second endpoint is \(B=(6,13)\).

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