Second Derivative Test

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AP Calculus AB › Second Derivative Test

Questions 1 - 10
1

At $x=-18$, $H'(-18)=0$ and $H''(-18)<0$; what does the Second Derivative Test conclude?

Cannot be determined from the information given.

Neither a maximum nor minimum because $H''(-18)<0$.

Local minimum at $x=-18$.

Inflection point at $x=-18$.

Local maximum at $x=-18$.

Explanation

The Second Derivative Test classifies critical points by examining the concavity behavior at those locations. Given $H'(-18)=0$ (critical point) and $H''(-18)<0$ (negative second derivative), the function is concave down at $x=-18$. Downward concavity at a critical point produces a hill-shaped curve, confirming a local maximum. Choice B could attract students who associate negative values with minimums, but negative second derivative specifically creates the downward curvature characteristic of maximums. Use this test when both critical point status and concavity direction are clearly established.

2

At $x=-17$, $F'(-17)=0$ and $F''(-17)<0$; classify the critical point at $x=-17$.

Inflection point at $x=-17$.

Cannot be determined from the information given.

Local maximum at $x=-17$.

Local minimum at $x=-17$.

Neither a maximum nor minimum because $F'(-17)=0$.

Explanation

The Second Derivative Test determines critical point nature through concavity analysis at those points. Since $F'(-17)=0$ establishes a critical point and $F''(-17)<0$ indicates negative concavity, the function is concave down at $x=-17$. Concave down behavior at a critical point forms a hill-shaped curve, indicating a local maximum. Choice A could confuse students who think negative signs correspond to minimums, but negative second derivative specifically means downward concavity (maximum). Apply this test when you have verified both the critical point condition and the sign of the second derivative.

3

A function has $f'(-1)=0$ and $f''(-1)<0$; how is the critical point at $x=-1$ classified?

Neither a maximum nor minimum because $f''(-1)<0$.

Local minimum at $x=-1$.

Local maximum at $x=-1$.

Inflection point at $x=-1$.

Cannot be determined from the information given.

Explanation

The Second Derivative Test uses concavity information to classify critical points where the first derivative equals zero. Since $f'(-1)=0$ establishes a critical point and $f''(-1)<0$ indicates negative concavity, the function is concave down at $x=-1$. Concave down behavior at a critical point creates a hill-like shape, confirming a local maximum. Choice A might seem appealing since negative values can be associated with minimums, but negative second derivative specifically means downward concavity (maximum). Remember that the Second Derivative Test requires both conditions: critical point and definitive concavity sign.

4

A differentiable function satisfies $p'(3)=0$ and is concave up at $x=3$; classify $x=3$.

Local maximum at $x=3$.

Local minimum at $x=3$.

Neither a maximum nor minimum because concavity is irrelevant.

Cannot be determined from the information given.

Inflection point at $x=3$.

Explanation

The Second Derivative Test utilizes concavity information to determine the type of critical point present. Since $p'(3)=0$ establishes a critical point and the function is concave up at $x=3$ (meaning $p''(3)>0$), the graph curves upward at this location. Upward concavity at a critical point creates a valley-shaped curve, confirming a local minimum. Choice A might attract students who confuse upward curvature with maximum behavior, but concave up specifically indicates the valley formation of minimums. Apply this test when both critical point and concavity direction are definitively known.

5

Given $g'(5)=0$ and $g''(5)>0$, what does the Second Derivative Test conclude at $x=5$?

Inflection point at $x=5$.

Cannot be determined because $g'(5)=0$.

Local minimum at $x=5$.

Local maximum at $x=5$.

No extremum because $g''(5)>0$.

Explanation

The Second Derivative Test determines the nature of critical points by analyzing the concavity at those locations. With $g'(5)=0$ confirming a critical point and $g''(5)>0$ indicating positive concavity, the graph is concave up at $x=5$. Concave up curvature at a critical point forms a valley, establishing a local minimum. Choice B could mislead students who associate positive values with maximums, but positive second derivative specifically indicates upward concavity (minimum). The test is applicable when both the critical point condition and second derivative sign are clearly determined.

6

At $x=16$, $G'(16)=0$ and $G$ is concave up at $x=16$; classify $x=16$.

Neither a maximum nor minimum because $G'(16)=0$.

Local minimum at $x=16$.

Inflection point at $x=16$.

Cannot be determined from the information given.

Local maximum at $x=16$.

Explanation

The Second Derivative Test determines the nature of critical points through concavity analysis. Since $G'(16)=0$ establishes a critical point and the function is concave up at $x=16$ (meaning $G''(16)>0$), the graph curves upward at this point. Upward concavity at a critical point forms a valley-shaped curve, indicating a local minimum. Choice B might confuse students who think upward curvature suggests maximums, but concave up specifically describes the valley formation of minimums. Apply this test when you can verify both critical point existence and the direction of concavity.

7

A twice-differentiable function $q$ satisfies $q'(5)=0$ and $q''(5)=2$; classify $x=5$.

An inflection point at $x=5$.

No conclusion can be drawn because $q''(5)>0$.

A local minimum at $x=5$.

Neither a maximum nor minimum because $q'(5)=0$.

A local maximum at $x=5$.

Explanation

To classify this critical point, we systematically apply the Second Derivative Test. With q'(5) = 0, we confirm x = 5 is a critical point, and since q''(5) = 2 > 0, the function exhibits upward concavity at this point. The combination of a horizontal tangent and positive concavity creates a local minimum—imagine a smooth valley or bowl shape centered at x = 5. The function decreases as it approaches x = 5 from either side, then increases as it moves away. Choice C incorrectly suggests that a positive second derivative prevents drawing conclusions, but the Second Derivative Test explicitly states that f''(c) > 0 at a critical point indicates a local minimum. Remember: at critical points, positive concavity (f''(c) > 0) means minimum, negative concavity (f''(c) < 0) means maximum.

8

For a differentiable function $f$, if $f'(2)=0$ and $f''(2)=-5$, what occurs at $x=2$?

A local maximum at $x=2$.

No conclusion can be drawn because $f'(2)=0$.

An inflection point at $x=2$.

A local minimum at $x=2$.

Neither a maximum nor minimum because $f''(2)<0$.

Explanation

This problem requires applying the Second Derivative Test to classify a critical point. Since f'(2) = 0, we have a critical point at x = 2, and since f''(2) = -5 < 0, the function is concave down at this point. When a function has a horizontal tangent line (f'(2) = 0) and is concave down (f''(2) < 0), the critical point must be a local maximum. The graph curves downward like an upside-down bowl at x = 2, creating a peak. Choice E incorrectly suggests that a negative second derivative prevents classification, but the Second Derivative Test specifically uses the sign of f''(c) to determine whether a critical point is a maximum or minimum. Remember: at a critical point, if f''(c) < 0, then it's a local maximum; if f''(c) > 0, then it's a local minimum.

9

Let $r$ be twice differentiable with $r'(-4)=0$ and $r''(-4)=-9$; classify the point $x=-4$.

A local maximum at $x=-4$.

An inflection point at $x=-4$.

Neither a maximum nor minimum because $r''(-4)<0$.

No conclusion can be drawn because $r'(-4)=0$.

A local minimum at $x=-4$.

Explanation

This question requires applying the Second Derivative Test to classify a critical point with negative concavity. Given r'(-4) = 0, we have a critical point at x = -4, and with r''(-4) = -9 < 0, the function is concave down at this location. When a critical point occurs where the function is concave down, it creates a local maximum—visualize an inverted bowl or mountain peak. The strongly negative value -9 indicates pronounced downward curvature, forming a clear peak at x = -4. Choice E incorrectly claims that a negative second derivative prevents classification as a maximum or minimum, but the Second Derivative Test specifically uses f''(c) < 0 to identify local maxima at critical points. Quick test tip: "negative second derivative = frowning graph = local maximum."

10

If $v'(-2)=0$ and $v$ is concave down at $x=-2$, what is the classification of $x=-2$?

A local minimum at $x=-2$.

Neither a maximum nor minimum because $v$ is concave down.

An inflection point at $x=-2$.

No conclusion can be drawn because $v'(-2)=0$.

A local maximum at $x=-2$.

Explanation

To classify this critical point, we apply the Second Derivative Test using concavity information. Given v'(-2) = 0, we have a critical point at x = -2, and since v is concave down at this point, we know v''(-2) < 0. The combination of a horizontal tangent and downward concavity creates a local maximum—imagine the peak of a mountain or the top of an arch. The function increases to reach its highest point at x = -2, then decreases on both sides. Choice D incorrectly suggests that being concave down prevents the point from being a maximum, but the Second Derivative Test specifically states that negative concavity (f''(c) < 0) at a critical point indicates a local maximum. Test tip: "concave down" means "negative second derivative," which always yields a local maximum at critical points.

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