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Sketch a function from its derivatives

Learning goals

  • Determines increasing and decreasing intervals from the sign of the first derivative.
  • Determines intervals of concavity using derivative information.
  • Finds and classifies local extrema using derivatives.
  • Verifies an actual change in concavity before declaring an inflection point.
  • Accounts for intercepts, asymptotes, or endpoint information when given.
  • Sketches a function consistent with its known values and derivatives.

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Sketch a function from its derivatives 1
Instead of a freehand sketch, give an exact **graph blueprint**. A differentiable function satisfies \(f(0)=1\), \(f'(x)<0\) for \(x<0\), \(f'(0)=0\), \(f'(x)>0\) for \(x>0\), and \(f''(x)>0\) for every real \(x\). Describe the graph from left to right. Your blueprint must state the monotonicity and concavity on each side of \(x=0\), and identify the type and coordinates of the point at \(x=0\).

Hints

- The sign of \(f'\) gives the direction of travel. - A negative-to-positive change in \(f'\) gives a local minimum. - The sign of \(f''\) gives concavity. - Use \(f(0)=1\) to place the landmark vertically.

Solution

1. Since \(f'(x)<0\) for \(x<0\), the graph decreases as it approaches \(x=0\) from the left. 2. Since \(f'(x)>0\) for \(x>0\), the graph increases to the right of \(x=0\). Thus \(f'\) changes from negative to positive, so \(x=0\) is a local minimum. 3. The anchor \(f(0)=1\) makes the minimum point \((0,1)\). 4. Because \(f''(x)>0\) everywhere, both the left and right branches are concave up. Blueprint: decreasing and concave up to \((0,1)\), then increasing and concave up.

Answer

Decreasing and concave up for \(x<0\); local minimum at \((0,1)\); increasing and concave up for \(x>0\).
Sketch a function from its derivatives 2
The graph of \(f'\) is shown. Describe the shape of a possible graph of \(f\): where does it rise, where does it fall, and what happens at \(x=2\)?
Figure for problem 562367

Hints

- Above the x-axis, the derivative has what sign? - Translate a derivative sign into the direction of the original graph. - When the sign changes at a zero, what shape does the original graph have nearby?

Solution

1. The derivative \(f'(x)=2-x\) is positive for \(x<2\) and negative for \(x>2\). 2. Therefore, a graph of \(f\) rises up to \(x=2\) and falls after \(x=2\). 3. The graph has a local maximum at \(x=2\). Its vertical position is not determined by \(f'\).

Answer

A graph of \(f\) rises for \(x<2\), has a local maximum at \(x=2\), and falls for \(x>2\). The height of the graph cannot be determined from \(f'\) alone.
Sketch a function from its derivatives 3
The graph of \(f'\) is a horizontal line at \(y=-2\). Describe the shape of the graph of \(f\). Can its exact height be determined?
Figure for problem 562368

Hints

- What does a horizontal derivative graph say about the slope of the original graph? - Does a constant slope make a curve or a straight line? - Could two vertically shifted straight lines share the same derivative?

Solution

1. The derivative is the constant \(f'(x)=-2\), so the slope of \(f\) is always \(-2\). 2. A function with this derivative has the form \(f(x)=-2x+C\), where \(C\) is any constant. 3. Thus its graph is a straight line sloping downward. The derivative graph does not determine \(C\) or the line's height.

Answer

The graph of \(f\) is a decreasing straight line with slope \(-2\). Its height is not determined.
Sketch a function from its derivatives 4
Instead of a freehand sketch, give an exact **graph blueprint**. For a differentiable function \(f\): - \(f'(x)>0\) for \(x<-2\), \(f'(x)<0\) for \(-2<x<1\), and \(f'(x)>0\) for \(x>1\); - \(f''(x)<0\) for \(x<0\) and \(f''(x)>0\) for \(x>0\); - \(f(-2)=4\), \(f(0)=1\), and \(f(1)=0\). Write the graph blueprint from left to right, splitting whenever monotonicity or concavity changes. Name every local extremum and inflection point with coordinates.

Hints

- Combine the sign of \(f'\) and the sign of \(f''\) on each interval. - Split the blueprint at \(-2\), \(0\), and \(1\). - Use the supplied function values only to place the named landmarks.

Solution

1. On \(( -\infty,-2)\), \(f'>0\) and \(f''<0\): increasing, concave down. 2. At \(x=-2\), \(f'\) changes positive to negative, so \((-2,4)\) is a local maximum. 3. On \((-2,0)\), \(f'<0\) and \(f''<0\): decreasing, concave down. 4. At \(x=0\), concavity changes from down to up, so \((0,1)\) is an inflection point. 5. On \((0,1)\), \(f'<0\) and \(f''>0\): decreasing, concave up. 6. At \(x=1\), \(f'\) changes negative to positive, so \((1,0)\) is a local minimum. 7. On \((1,\infty)\), \(f'>0\) and \(f''>0\): increasing, concave up.

Answer

Increasing/concave down on \(( -\infty,-2)\); local max \((-2,4)\); decreasing/concave down on \((-2,0)\); inflection \((0,1)\); decreasing/concave up on \((0,1)\); local min \((1,0)\); increasing/concave up on \((1,\infty)\).
Sketch a function from its derivatives 5
A curve has the following known features on \((-3,2)\). <table><tr><th>Interval</th><th>Sign of \(f'\)</th><th>Sign of \(f''\)</th></tr><tr><td>\((-3,-2)\)</td><td>Positive</td><td>Negative</td></tr><tr><td>\((-2,0)\)</td><td>Negative</td><td>Negative</td></tr><tr><td>\((0,1)\)</td><td>Negative</td><td>Positive</td></tr><tr><td>\((1,2)\)</td><td>Positive</td><td>Positive</td></tr></table> The curve is continuous, has horizontal tangents at \(x=-2\) and \(x=1\), and passes through \((-2,4)\), \((0,2)\), and \((1,0)\). Describe a possible sketch: give the two turning points, the inflection-point x-coordinate, and the directions and concavities in the four intervals.

Hints

- Read the two derivative signs in each row as two different graph features. - A turning point depends on a change in the first derivative's sign. - An inflection point depends on a change in concavity. - Use the supplied points only after deciding which kind of feature each one has.

Solution

1. Positive to negative \(f'\) at \(-2\) gives a local maximum at \((-2,4)\). 2. Negative to positive \(f'\) at \(1\) gives a local minimum at \((1,0)\). 3. The sign of \(f''\) changes from negative to positive at \(x=0\), so \((0,2)\) is an inflection point. 4. The first interval is increasing and concave down; the second is decreasing and concave down; the third is decreasing and concave up; the fourth is increasing and concave up.

Answer

Local maximum \((-2,4)\); local minimum \((1,0)\); inflection point \((0,2)\). On \((-3,-2)\): increasing, concave down. On \((-2,0)\): decreasing, concave down. On \((0,1)\): decreasing, concave up. On \((1,2)\): increasing, concave up.
Sketch a function from its derivatives 6
The graph of the derivative \(f^{\prime}\) is shown. 1. Describe the key features of one possible graph of \(f\), including its local extrema and inflection point. 2. Describe where \(f\) is increasing and decreasing on \([-3, 5]\).
Figure for problem 533986

Hints

- Zeros of \(f^{\prime}\) give locations of horizontal tangents on \(f\). - Where \(f^{\prime}>0\), \(f\) increases. - Where \(f^{\prime}<0\), \(f\) decreases. - Where \(f^{\prime}\) changes from increasing to decreasing, \(f\) changes concavity.

Solution

1. The derivative is \(0\) at \(x=-2\) and \(x=4\). It changes from negative to positive at \(x=-2\), so a possible graph of \(f\) has a local minimum there. It changes from positive to negative at \(x=4\), so the graph has a local maximum there. The graph of \(f^{\prime}\) increases up to \(x=1\) and decreases after \(x=1\), so \(f\) changes from concave up to concave down and has an inflection point at \(x=1\). Any vertical shift with these slope and concavity properties is acceptable. 2. Since \(f^{\prime}<0\) on \((-3, -2)\), \(f\) is decreasing there. Since \(f^{\prime}>0\) on \((-2, 4)\), \(f\) is increasing there. Since \(f^{\prime}<0\) on \((4, 5)\), \(f\) is decreasing there.

Answer

1. One possible graph has a local minimum at \(x=-2\), an inflection point at \(x=1\), and a local maximum at \(x=4\), with slopes matching the displayed derivative. 2. Decreasing on \((-3, -2)\) and \((4, 5)\); increasing on \((-2, 4)\)
Sketch a function from its derivatives 7
The graph of the derivative \(f^{\prime}\) is shown on \([-2, 5]\). 1. Describe the key features of one possible graph of \(f\) on the displayed interval. 2. At \(x=3\), the graph of \(f^{\prime}\) touches the x-axis without changing sign. What special feature does the graph of \(f\) have there? Briefly justify your answer.
Figure for problem 533987

Hints

- A zero of the derivative without a sign change does not produce a local extremum. - Use whether \(f^{\prime}\) is increasing or decreasing to determine the concavity of \(f\). - Determine whether \(f^{\prime}\) is increasing or decreasing on each side of \(x=3\). - A horizontal tangent together with a change in concavity indicates a stationary inflection point.

Solution

1. The derivative changes from negative to positive at \(x=0\), so a possible graph of \(f\) has a local minimum at \(x=0\). The graph of \(f^{\prime}\) increases up to \(x=1\), decreases from \(x=1\) to \(x=3\), and increases after \(x=3\). Therefore, \(f\) is concave up, then concave down, then concave up, with inflection points at \(x=1\) and \(x=3\). At \(x=3\), \(f^{\prime}=0\), but \(f^{\prime}\) is positive on both sides, so \(f\) remains increasing and has no local extremum there. 2. The graph of \(f^{\prime}\) has a local minimum at \(x=3\). Therefore, \(f^{\prime}\) changes from decreasing to increasing, so the concavity of \(f\) changes. Because \(f^{\prime}(3)=0\), \(f\) has a stationary inflection point at \(x=3\).

Answer

1. One possible graph has a local minimum at \(x=0\), an inflection point at \(x=1\), and a stationary inflection point at \(x=3\); it remains increasing through \(x=3\). 2. At \(x=3\), \(f\) has a stationary inflection point: its tangent is horizontal, but the function remains increasing through the point.
Sketch a function from its derivatives 8
The graph of the derivative \(f^{\prime}\) is shown on \([-4, 3]\). 1. Describe the key features of one possible graph of \(f\) on the displayed interval. 2. Find every x-value where the tangent to \(f\) is horizontal. Classify each point as a local maximum, a local minimum, or a stationary inflection point, and justify your classifications.
Figure for problem 533989

Hints

- Find the zeros of \(f^{\prime}\). - Use the sign of \(f^{\prime}\) on each interval to determine whether \(f\) increases or decreases. - A sign change at a zero indicates a local extremum. - Turning points of \(f^{\prime}\) indicate changes in the concavity of \(f\).

Solution

1. The zeros of \(f^{\prime}\) are \(x=-3\), \(x=-1\), and \(x=2\). These are the x-values where \(f\) has horizontal tangents. The graph of \(f^{\prime}\) has a local maximum at approximately \(x=-2.1\) and a local minimum at approximately \(x=0.8\), so a corresponding graph of \(f\) changes concavity at those x-values. Any vertical shift with these slope and concavity properties is possible. 2. At \(x=-3\), \(f^{\prime}\) changes from negative to positive, so \(f\) has a local minimum. At \(x=-1\), it changes from positive to negative, so \(f\) has a local maximum. At \(x=2\), it changes from negative to positive, so \(f\) has a local minimum. There are no stationary inflection points because the derivative changes sign at every zero.

Answer

1. One possible graph has local minima at \(x=-3\) and \(x=2\), a local maximum at \(x=-1\), and inflection points at approximately \(x=-2.1\) and \(x=0.8\). 2. Horizontal tangents occur at \(x=-3\) (local minimum), \(x=-1\) (local maximum), and \(x=2\) (local minimum). There are no stationary inflection points.
Sketch a function from its derivatives 9
The graph shown is \(f'\), the derivative of a cubic polynomial \(f\). Give a text-only graph blueprint for \(f\). a) Find the x-coordinates of all local extrema of \(f\) and classify them. b) State the maximal intervals on which \(f\) is increasing and decreasing. c) State the maximal intervals on which \(f\) is concave up and concave down, and give the inflection-point x-coordinate. d) List the significant x-values from left to right and describe the resulting sequence of graph features.
Figure for problem 534562

Hints

- First use the zeros and sign of \(f'\) to organize the monotonicity and extrema. - The original function is concave up where its derivative graph is increasing and concave down where the derivative graph is decreasing. - The vertex of the derivative graph supplies the inflection input of the cubic. - Put the three significant x-values in order before writing the final blueprint.

Solution

1. The zeros of \(f'\) are \(x=-1\) and \(x=3\). The derivative is positive for \(x<-1\), negative for \(-1<x<3\), and positive for \(x>3\). Hence \(f\) has a local maximum at \(x=-1\) and a local minimum at \(x=3\). 2. The same sign chart gives increasing intervals \(( -\infty,-1)\) and \((3,\infty)\), with decreasing interval \((-1,3)\). 3. The graph of \(f'\) decreases until its vertex at \(x=1\) and increases afterward. Thus \(f''<0\) for \(x<1\) and \(f''>0\) for \(x>1\). Therefore, \(f\) is concave down on \(( -\infty,1)\), concave up on \((1,\infty)\), and has an inflection point at \(x=1\). 4. Left to right, the graph increases concave down to a local maximum at \(x=-1\), decreases concave down to the inflection input \(x=1\), continues decreasing concave up to a local minimum at \(x=3\), then increases concave up.

Answer

a) Local maximum at \(x=-1\); local minimum at \(x=3\). b) Increasing on \(( -\infty,-1)\) and \((3,\infty)\); decreasing on \((-1,3)\). c) Concave down on \(( -\infty,1)\); concave up on \((1,\infty)\); inflection-point x-coordinate \(1\). d) Left-to-right features: maximum at \(-1\), inflection at \(1\), minimum at \(3\), with the monotonicity/concavity sequence stated above.
Sketch a function from its derivatives 10
Give an exact **graph blueprint** for a differentiable function \(f\). You are given \(f'(x)=-(x+1)^2(x-2)\), together with \(f(-1)=0\), \(f(1)=4\), and \(f(2)=\tfrac{27}{4}\). a) Classify the horizontal tangencies at \(x=-1\) and \(x=2\) from the sign of \(f'\). b) Differentiate \(f'\) to find every inflection-point x-coordinate. c) Give the complete left-to-right monotonicity/concavity blueprint and name each supplied landmark with coordinates.

Hints

- A squared factor in \(f'\) gives a zero where the sign of \(f'\) may stay the same. - Differentiate the supplied expression for \(f'\) to get \(f''\); do not integrate. - Split the blueprint at every x-value where either monotonicity or concavity changes.

Solution

1. Since \(f'(x)=-(x+1)^2(x-2)\), the squared factor does not change sign at \(x=-1\). Thus \(f'>0\) on both sides of \(-1\). At \(x=2\), \(f'\) changes from positive to negative. 2. Therefore \((-1,0)\) is not a local extremum, while \((2,\tfrac{27}{4})\) is a local maximum. 3. Differentiate the given derivative: \(f''(x)=-3x^2+3=3(1-x^2)\). Hence \(f''=0\) at \(x=-1\) and \(x=1\), and its sign changes at both values. 4. At \(x=-1\), the horizontal tangent together with the concavity change makes \((-1,0)\) a stationary inflection point. At \(x=1\), \((1,4)\) is a nonstationary inflection point. 5. The graph is increasing/concave down on \(( -\infty,-1)\), increasing/concave up on \((-1,1)\), increasing/concave down on \((1,2)\), and decreasing/concave down on \((2,\infty)\).

Answer

Stationary inflection \((-1,0)\); nonstationary inflection \((1,4)\); local maximum \((2,\tfrac{27}{4})\). Blueprint: ↑/concave down on \(( -\infty,-1)\), ↑/concave up on \((-1,1)\), ↑/concave down on \((1,2)\), and ↓/concave down on \((2,\infty)\).
Sketch a function from its derivatives 11
On the closed interval \([-4,4]\), give an exact **graph blueprint** for \(f\). The derivative signs are: \(f'(x)>0\) on \((-4,-1)\), \(f'(-1)=0\), \(f'(x)<0\) on \((-1,2)\), \(f'(2)=0\), and \(f'(x)>0\) on \((2,4)\). Also, \(f''(x)<0\) on \((-4,0)\) and \(f''(x)>0\) on \((0,4)\). Function values are \(f(-4)=0\), \(f(-1)=5\), \(f(0)=3\), \(f(2)=1\), and \(f(4)=4\). Give the segment-by-segment blueprint, classify the two interior extrema and the inflection point, and identify the absolute maximum and minimum on \([-4,4]\).

Hints

- Use all breakpoints where either derivative sign changes. - Endpoint values matter for absolute extrema even though endpoints are not interior critical numbers. - Keep local classification and absolute comparison as separate steps.

Solution

1. \((-4,-1)\): increasing and concave down. 2. At \((-1,5)\), \(f'\) changes positive to negative, so there is a local maximum. 3. \((-1,0)\): decreasing and concave down. 4. At \((0,3)\), concavity changes down to up, so this is an inflection point. 5. \((0,2)\): decreasing and concave up. 6. At \((2,1)\), \(f'\) changes negative to positive, so there is a local minimum. 7. \((2,4)\): increasing and concave up. 8. Compare the supplied candidate values at the endpoints and local extrema: \(0,5,1,4\). The absolute maximum is \(5\) at \(x=-1\); the absolute minimum is \(0\) at the endpoint \(x=-4\).

Answer

Blueprint: ↑/concave down on \((-4,-1)\); local max \((-1,5)\); ↓/concave down to inflection \((0,3)\); ↓/concave up to local min \((2,1)\); ↑/concave up on \((2,4)\). Absolute max \(5\) at \(x=-1\); absolute min \(0\) at \(x=-4\).
Sketch a function from its derivatives 12
A differentiable function satisfies \(f'(x)=-(x+2)(x-1)\). It is known that \(f(-2)=0\), \(f(-\frac{1}{2})=\frac{9}{4}\), and \(f(1)=\frac{9}{2}\). Without finding a formula for \(f\), give a graph blueprint: find the local extrema and the inflection point, and state where the graph increases, decreases, is concave up, and is concave down.

Hints

- Compare the signs of the factored first derivative on the intervals separated by its zeros. - Differentiate the given derivative to check where the curve changes concavity. - The supplied function values place each turning or inflection point vertically; no formula for the original function is needed.

Solution

1. The zeros of \(f'\) are \(x=-2\) and \(x=1\). The factored derivative is negative on \((-\infty,-2)\), positive on \((-2,1)\), and negative on \((1,\infty)\). 2. Therefore, \((-2,0)\) is a local minimum and \((1,\frac{9}{2})\) is a local maximum. 3. Differentiate: \(f''(x)=-2x-1\). This is positive for \(x<-\frac{1}{2}\) and negative for \(x>-\frac{1}{2}\), so \((-\frac{1}{2},\frac{9}{4})\) is an inflection point. 4. From left to right, the graph decreases and is concave up before \(x=-2\); increases and is concave up between \(-2\) and \(-\frac{1}{2}\); increases and is concave down between \(-\frac{1}{2}\) and \(1\); then decreases and is concave down after \(x=1\).

Answer

Local minimum: \((-2,0)\). Inflection point: \((-\frac{1}{2},\frac{9}{4})\). Local maximum: \((1,\frac{9}{2})\). Decreasing and concave up on \((-\infty,-2)\); increasing and concave up on \((-2,-\frac{1}{2})\); increasing and concave down on \((-\frac{1}{2},1)\); decreasing and concave down on \((1,\infty)\).
Sketch a function from its derivatives 13
Let \(f\) be differentiable with \(f(0)=0\) and \(f'(x)=(x^2-1)(x^2-4)\). The end behavior is \(f(x)\to-\infty\) as \(x\to-\infty\) and \(f(x)\to\infty\) as \(x\to\infty\). Give a complete exact graph blueprint without finding a formula for \(f\): a) classify all four critical numbers; b) differentiate \(f'\) and find all inflection-point x-coordinates; c) state every maximal monotonicity and concavity interval; and d) list the significant x-values in left-to-right order, including the anchor \((0,0)\), so another student could sketch the graph from your blueprint.

Hints

- Use the ordered simple zeros of \(f'\) to build its sign chart. - Differentiate \(f'\) before building the concavity chart. - Merge the two ordered breakpoint lists only after both charts are correct. - The anchor and stated end behavior place the qualitative graph; no integration is needed.

Solution

1. The critical numbers are \(-2,-1,1,2\). Because all four zeros of \(f'\) are simple, its sign alternates. Since \(f'>0\) for large \(|x|\), the sign pattern is \(+,-,+,-,+\). 2. Therefore \(x=-2\) and \(x=1\) are local maxima, while \(x=-1\) and \(x=2\) are local minima. The maximal increasing intervals are \(( -\infty,-2)\), \((-1,1)\), and \((2,\infty)\); the maximal decreasing intervals are \((-2,-1)\) and \((1,2)\). 3. Differentiate: \(f''(x)=4x^3-10x=2x(2x^2-5)\). Thus the inflection inputs are \(-\sqrt{5/2},0,\sqrt{5/2}\). Each zero is simple, so concavity changes at each one. 4. Testing signs gives concave down on \(( -\infty,-\sqrt{5/2})\), concave up on \((-\sqrt{5/2},0)\), concave down on \((0,\sqrt{5/2})\), and concave up on \((\sqrt{5/2},\infty)\). 5. Left-to-right significant inputs are \(-2,-\sqrt{5/2},-1,0,1,\sqrt{5/2},2\). At \(x=0\), the graph passes through \((0,0)\) and changes concavity. Combine this order with the stated end behavior and interval directions.

Answer

Local maxima at \(x=-2,1\); local minima at \(x=-1,2\). Increasing on \(( -\infty,-2)\), \((-1,1)\), \((2,\infty)\); decreasing on \((-2,-1)\), \((1,2)\). Inflection inputs \(-\sqrt{5/2},0,\sqrt{5/2}\). Concave down on \(( -\infty,-\sqrt{5/2})\) and \((0,\sqrt{5/2})\); concave up on \((-\sqrt{5/2},0)\) and \((\sqrt{5/2},\infty)\). Significant order: \(-2,-\sqrt{5/2},-1,(0,0),1,\sqrt{5/2},2\), with left end down and right end up.

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