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Problem 10: Let \( f(x)=\frac{1}{x} \) on the interval \( [1,4] \). - Use the MVT to find the value(s) of \( c \) in \( (1,4) \) where \( f^{\prime}(c)=\frac{f(4)-f(1)}{4-1} \)

Ask by Phillips Montgomery. in the United States
Jan 23,2025

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Tutor-Verified Answer

Answer

The value of \( c \) is 2.

Solution

To apply the Mean Value Theorem (MVT) to the function \( f(x) = \frac{1}{x} \) on the interval \([1, 4]\), follow these steps: 1. **Verify the Conditions for MVT:** - **Continuity:** \( f(x) = \frac{1}{x} \) is continuous on \([1, 4]\). - **Differentiability:** \( f(x) = \frac{1}{x} \) is differentiable on \((1, 4)\). Since both conditions are satisfied, MVT applies. 2. **State the MVT:** There exists at least one \( c \) in \((1, 4)\) such that: \[ f'(c) = \frac{f(4) - f(1)}{4 - 1} \] 3. **Compute \( f(4) \) and \( f(1) \):** \[ f(4) = \frac{1}{4}, \quad f(1) = 1 \] 4. **Calculate the Right-Hand Side of MVT:** \[ \frac{f(4) - f(1)}{4 - 1} = \frac{\frac{1}{4} - 1}{3} = \frac{-\frac{3}{4}}{3} = -\frac{1}{4} \] 5. **Find \( f'(x) \):** \[ f'(x) = -\frac{1}{x^2} \] 6. **Set \( f'(c) \) Equal to \(-\frac{1}{4}\):** \[ -\frac{1}{c^2} = -\frac{1}{4} \implies \frac{1}{c^2} = \frac{1}{4} \] 7. **Solve for \( c \):** \[ c^2 = 4 \implies c = 2 \quad (\text{since } c \in (1, 4)) \] **Conclusion:** The value of \( c \) that satisfies the Mean Value Theorem for the given function on \([1, 4]\) is: \(\boxed{2}\)

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First, we need to find \( f(4) \) and \( f(1) \). For \( f(x) = \frac{1}{x} \), we have: - \( f(4) = \frac{1}{4} \) - \( f(1) = 1 \) Next, we calculate the average rate of change: \[ \frac{f(4) - f(1)}{4 - 1} = \frac{\frac{1}{4} - 1}{3} = \frac{-\frac{3}{4}}{3} = -\frac{1}{4} \] Now, we need to find \( f'(x) \). The derivative \( f'(x) \) is: \[ f'(x) = -\frac{1}{x^2} \] Setting \( f'(c) = -\frac{1}{4} \), we have: \[ -\frac{1}{c^2} = -\frac{1}{4} \] This simplifies to: \[ \frac{1}{c^2} = \frac{1}{4} \implies c^2 = 4 \implies c = 2 \quad (\text{since } c \in (1,4)) \] Thus, the value of \( c \) is \( 2 \).

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