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1. A function \( f \) is continuous on the closed interval \( [-2,5] \), differentiable on the open interval \( (-2,5) \), and \( f(-2)=f(5)=3 \). Which of the following statements must be true? (A) There is a number \( c \) in the interval \( (-2,5) \) for which \( f(c)=0 \). (B) \( f^{\prime}(x)>0 \) for all numbers in the interval \( (-2,5) \). (C) \( f^{\prime}(x)=3 \) for all numbers in the interval \( (-2,5) \). (D) None of the above The graph of \( f^{\prime} \) for the interval \( [-3,5] \) is shown below.

Ask by Harris Brewer. in the United States
Jan 22,2025

Real Tutor Solution

Tutor-Verified Answer

Answer

None of the statements (A), (B), or (C) must be true based on the given conditions.

Solution

To determine which statement must be true given the conditions, let's analyze each option: 1. **Given Conditions:** - \( f \) is continuous on the closed interval \([-2, 5]\). - \( f \) is differentiable on the open interval \((-2, 5)\). - \( f(-2) = f(5) = 3 \). 2. **Applying Rolle's Theorem:** Since \( f \) is continuous on \([-2, 5]\) and differentiable on \((-2, 5)\) with \( f(-2) = f(5) \), Rolle's Theorem guarantees that there exists at least one number \( c \) in \((-2, 5)\) where \( f'(c) = 0 \). 3. **Evaluating the Statements:** - **(A) There is a number \( c \) in the interval \((-2, 5)\) for which \( f(c) = 0 \).** - **Not necessarily true.** The function could maintain a constant value of 3 without ever reaching 0. - **(B) \( f'(x) > 0 \) for all numbers in the interval \((-2, 5)\).** - **Not true.** By Rolle's Theorem, there exists at least one \( c \) where \( f'(c) = 0 \), which contradicts the statement. - **(C) \( f'(x) = 3 \) for all numbers in the interval \((-2, 5)\).** - **Not necessarily true.** The derivative could vary and is only required to equal zero at some point by Rolle's Theorem. 4. **Conclusion:** Since none of the statements (A), (B), or (C) must be true based on the given conditions, the correct answer is: **Answer:** (D) None of the above

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The Deep Dive

The function \( f \) is supported by the fact that it is continuous and returns to the same value at the endpoints: \( f(-2) = f(5) = 3 \). This suggests that the function must oscillate at some point in the interval \( (-2, 5) \). Specifically, the Mean Value Theorem tells us that since \( f(-2) = f(5) \), there exists at least one point \( c \) in \( (-2, 5) \) where \( f'(c) = 0 \). This means there must be a maximum or minimum within that interval. For option (D), it's actually correct to choose "None of the above" because none of the other options hold true based on the given conditions: (A) does not necessarily have to be valid, and (B) & (C) imply strict behaviors for \( f' \) which are not guaranteed just because \( f(-2) = f(5) \).

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