Extreme Value Theorem, Extrema, Critical Points

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AP Calculus BC › Extreme Value Theorem, Extrema, Critical Points

Questions 1 - 10
1

Let $f$ be continuous on $0,3$ and differentiable on $(0,3)$ with exactly one critical point $c$ in $(0,3)$; which statement must be true?

Both absolute extrema must occur at $c$.

Neither absolute extremum can occur at an endpoint.

$f$ has a local extremum at $c$.

$f$ attains both an absolute maximum and an absolute minimum on $[0,3]$.

$f$ has exactly one absolute extremum on $[0,3]$.

Explanation

This question tests the Extreme Value Theorem (EVT) and the role of a single critical point in determining extrema. The function f is continuous on [0,3], so by the EVT, it attains both an absolute maximum and an absolute minimum on the interval. The single critical point c in (0,3) is a candidate for a local extremum, but absolute extrema could also occur at the endpoints. Continuity on the closed interval ensures the existence of these global extrema, regardless of whether the critical point is a max, min, or neither. A tempting distractor is choice A, which claims a local extremum at c, but this fails because f'(c) = 0 does not guarantee one, as in $x^3$ where it's an inflection point. To apply the EVT, check continuity on a closed, bounded interval; locate all critical points and endpoints, then evaluate the function to determine the absolute extrema.

2

A continuous function $s$ on $2,8$ has $s'(x)<0$ on $(2,5)$ and $s'(x)>0$ on $(5,8)$. Which is true?

$s$ must be constant on $[2,8]$ since the derivative changes sign.

$s$ has an absolute maximum at $x=5$.

$s$ has a local minimum at $x=5$, but it may not be absolute.

$s$ has no extrema because $s'(x)$ is never $0$ on $(2,8)$.

$s$ has an absolute minimum at $x=5$.

Explanation

This problem combines derivative sign analysis with extrema identification. Since s is continuous on [2,8] with s'(x)<0 on (2,5) (decreasing) and s'(x)>0 on (5,8) (increasing), the function changes from decreasing to increasing at x=5, making it a local minimum. Moreover, since s decreases until x=5 and then increases afterward, this local minimum at x=5 must also be the absolute minimum on the entire interval [2,8]. Option A incorrectly identifies this as a maximum rather than minimum—when derivatives change from negative to positive, we have a minimum. The sign change rule: negative to positive derivative = minimum point.

3

Let $r$ be continuous on $0,4$ and satisfy $r(0)=2$, $r(4)=2$, and $r(x)>2$ for $0<x<4$; which statement is true?

$r$ must have an absolute maximum at an endpoint.

$r$ must have a critical point at $x=0$.

$r$ has no absolute minimum because it is never less than $2$.

$r$ has a local maximum at $x=0$.

$r$ has an absolute minimum at both endpoints $x=0$ and $x=4$.

Explanation

This question explores the Extreme Value Theorem (EVT), extrema at endpoints, and function behavior on a closed interval. The function r is continuous on [0,4], so the EVT guarantees absolute extrema exist, with the minimum value of 2 attained at both endpoints x=0 and x=4, since r(x) > 2 inside. There must be critical points inside where r' = 0 or undefined, as the function rises above 2 and returns, indicating local maxima or minima. The absolute minimum occurs at the endpoints, highlighting that extrema can be at boundaries even without critical points there. A tempting distractor is choice C, which claims no absolute minimum because r is never less than 2, but this fails as it attains 2 at the endpoints, which is the minimum. For the EVT checklist, verify continuity on a closed interval; find critical points in the interior, evaluate at them and endpoints, and identify the highest and lowest values.

4

Let $f$ be continuous on $0,4$ with $f(0)=2,f(1)=5,f(2)=1,f(3)=4,f(4)=3$. Which is true?

$f$ must have an absolute maximum value $5$ and an absolute minimum value $1$.

$f$ must have an absolute maximum at $x=1$ and an absolute minimum at $x=2$.

EVT guarantees absolute extrema, but their $x$-locations cannot be determined from the data.

$f$ has no absolute extrema because the given data are only at five points.

$f$ cannot have absolute extrema without knowing $f'(x)$.

Explanation

This question tests EVT application when only discrete function values are known. Since f is continuous on the closed interval [0,4], the Extreme Value Theorem guarantees that f must attain both an absolute maximum and an absolute minimum somewhere on this interval. From the given values, we can see that 5 is the largest and 1 is the smallest, but we cannot determine where the actual absolute extrema occur because f might attain even larger values between the given points or the same extreme values at multiple locations. Option A incorrectly assumes the given values are the only possible values f can take. The EVT guarantee: extrema exist, but their exact locations require complete function information.

5

Let $f$ be continuous on $-2,3$ with $f'(-1)=0$ and $f'(1)$ undefined. Which statement must be true?

$f$ has no absolute extrema because $f'(1)$ is undefined.

$f$ has both an absolute maximum and an absolute minimum on $[-2,3]$.

$f$ has an absolute maximum at $x=-1$ because $f'(-1)=0$.

$f$ has exactly two critical points, so it has exactly two absolute extrema.

$f$ has an absolute minimum at an interior critical point, not at an endpoint.

Explanation

This question tests the Extreme Value Theorem (EVT), which guarantees absolute extrema for continuous functions on closed intervals. Since f is continuous on the closed interval [-2,3], EVT ensures that f must attain both an absolute maximum and an absolute minimum somewhere on this interval. The critical points at x=-1 (where f'(-1)=0) and x=1 (where f'(1) is undefined) are candidates for these extrema, along with the endpoints x=-2 and x=3. Option B incorrectly assumes the absolute maximum must occur at x=-1 just because f'(-1)=0, but we'd need to compare values at all critical points and endpoints. Remember the EVT checklist: continuous function + closed interval = guaranteed absolute max and min.

6

A function $r$ is continuous on $-3,3$ and has exactly one critical point at $x=0$. Which statement must be true?

$r$ has both an absolute maximum value and an absolute minimum value on $[-3,3]$.

$r$ has an absolute minimum at $x=0$.

$r$ has an absolute maximum at $x=0$.

$r$ has no absolute extrema because it has only one critical point.

All absolute extrema must occur at critical points, not endpoints.

Explanation

This question tests the Extreme Value Theorem (EVT) and the impact of critical points on extrema existence. Continuity of r on [-3,3] guarantees, by EVT, both an absolute maximum and an absolute minimum on the closed interval. The single critical point at x=0 is a candidate for a local extremum, but absolute extrema could occur there or at endpoints x=-3 or x=3. Evaluating function values at the critical point and endpoints is required to determine the actual max and min. Choice B tempts by assuming the critical point is the absolute maximum, but it fails as endpoints might yield higher values depending on the function. EVT checklist: ensure continuity on closed bounded interval, locate critical points (f'=0 or undefined), compare values at those points and endpoints to identify extrema.

7

A function $p$ is continuous on $1,5$ and differentiable on $(1,5)$. Which statement is true?

If $p$ has an absolute minimum, it must occur at $x=1$.

If $p'(x)$ exists for all $x$ in $(1,5)$, $p$ has no absolute extrema on $[1,5]$.

Any absolute maximum of $p$ must occur at a point where $p'(x)=0$.

$p$ has both an absolute maximum value and an absolute minimum value on $[1,5]$.

If $p'(c)=0$, then $p(c)$ is an absolute extremum value.

Explanation

This question examines the Extreme Value Theorem (EVT) and properties of extrema for differentiable functions. Continuity on the closed interval [1,5] ensures, by EVT, that p attains both an absolute maximum and an absolute minimum value on the interval. Differentiability on (1,5) implies critical points where p'(x)=0 are possible interior candidates for extrema, but absolute extrema could also be at endpoints x=1 or x=5. The theorem guarantees existence without specifying locations, emphasizing evaluation at both critical points and boundaries. Choice B is a common distractor but incorrect because absolute maxima can occur at endpoints even if p'(x)=0 points exist interiorly. For EVT, follow this checklist: verify continuity on a closed bounded interval, locate critical points (where derivative is zero or undefined), and compare function values at those points and the endpoints.

8

A continuous function $g$ is defined on $0,4$ and satisfies $g'(2)=0$ and $g(0)=g(4)$. Which is true?

The absolute minimum value of $g$ must occur at $x=2$.

Because $g(0)=g(4)$, $g$ has no absolute extrema on $[0,4]$.

Since $g'(2)=0$, $x=2$ is an absolute extremum of $g$.

The absolute maximum value of $g$ must occur at $x=2$.

$g$ has an absolute maximum value on $[0,4]$ and an absolute minimum value on $[0,4]$.

Explanation

This question evaluates the Extreme Value Theorem (EVT) and the role of critical points in finding extrema. Since g is continuous on the closed interval [0,4], the EVT ensures it has both an absolute maximum and an absolute minimum on that interval. The critical point at x=2 where g'(2)=0 is a candidate for an extremum, but the absolute max and min could also occur at the endpoints x=0 or x=4, especially given g(0)=g(4). Evaluating all candidates—endpoints and critical points—is necessary to determine the actual extrema. Choice B is a tempting distractor but fails because the absolute maximum might occur at an endpoint if g(2) is not the largest value, as the critical point only guarantees a local extremum possibility. For EVT applications, use this checklist: confirm continuity, verify closed bounded interval, find critical points where derivative is zero or undefined, and compare values at those points and endpoints.

9

Let $m$ be continuous on $2,7$ with $m'(x)=0$ for all $x$ in $(2,7)$; which statement must be true?

$m$ has at least one critical point in $(2,7)$.

$m$ must have a local maximum in $(2,7)$.

$m$ attains an absolute maximum and an absolute minimum, both at endpoints.

$m$ has no absolute extrema on $[2,7]$.

$m$ is constant on $[2,7]$.

Explanation

This question assesses the Extreme Value Theorem (EVT) and consequences of no zero-derivative points on extrema. Since m is continuous on [2,7], the EVT guarantees absolute maximum and minimum values are attained on the interval. With m'(x) ≠ 0 everywhere in (2,7), there are no critical points where m' = 0, implying m is strictly monotonic, so extrema must be at the endpoints. Continuity ensures these extrema exist, and the lack of interior critical points confines them to the boundaries. A tempting distractor is choice B, which says there is at least one critical point in (2,7), but this fails because m' exists and is nonzero, so no such points. For applying the EVT, confirm continuity on a closed interval; if there are no interior critical points, compare values at the endpoints to find the absolute max and min.

10

Let $f(x)=|x-1|$ on $-2,4$; which statement about extrema and critical points is true?

$f$ has a local maximum at $x=1$ but no global minimum.

$f$ has no critical points because $f'(x)$ exists for all $x$.

$f$ has a global minimum at $x=1$, and $x=1$ is a critical point.

Endpoints cannot be candidates for global extrema for $f$.

$f$ has a global maximum at $x=1$.

Explanation

This question probes understanding of the Extreme Value Theorem (EVT), extrema, and critical points for a specific function. For f(x) = |x-1| on [-2,4], which is continuous on the closed interval, the EVT guarantees absolute extrema exist. The function has a critical point at x=1 where f' does not exist (due to the corner), and evaluating shows f(1)=0 is the global minimum, while maxima are at the endpoints. Critical points and endpoints must be checked to locate these extrema, with the minimum occurring at the critical point in this case. A tempting distractor is choice B, which claims a global maximum at x=1, but this fails because x=1 is actually the minimum, as the function increases away from it on both sides. For the EVT checklist, confirm continuity on a closed interval; identify critical points (f' = 0 or undefined) and endpoints, then compare function values to find absolute max and min.

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