Thomas' Calculus 13th Edition

Published by Pearson
ISBN 10: 0-32187-896-5
ISBN 13: 978-0-32187-896-0

Chapter 4: Applications of Derivatives - Section 4.2 - The Mean Value Theorem - Exercises 4.2 - Page 197: 12

Answer

$f(x)$ satisfies the hypotheses (see reasons below)

Work Step by Step

The two hypotheses of the Mean Value Theorem are: (1) $y=f(x)$ is continuous over a closed interval $[a, b]$ and (2) $f(x)$ is differentiable on the interval's interior $(a, b)$. --- In this problem, (1) - the only issue with continuity could arise at $x=2.$ $\displaystyle \lim_{x\rightarrow 2^{-}}f(x)=\lim_{x\rightarrow 2^{-}}(2x-3)=1$ $\displaystyle \lim_{x\rightarrow 2^{+}}f(x)=\lim_{x\rightarrow 2^{+}}(6x-x^{2}-7)=12-4-7=1$ So $\displaystyle \lim_{x\rightarrow 2}f(x)$ exists and equals $f(2)=1$. f is continuous at $x=2 \Rightarrow$ hypothesis (1) is satisfied. $(2) $ Observe what happens with the derivative at $x=2.$ $\displaystyle \left.\frac{d}{dx}(2x-3)\right|_{x=2}=\left.(2)\right|_{x=2}=2$ $\displaystyle \left.\frac{d}{dx}(6x-x^{2}-7)\right|_{x=2}=\left.(6-2x)\right|_{x=2}=2$ This means that $\displaystyle \lim_{h\rightarrow 0}\frac{f(x+h)-f(-1)}{h}$ exists (one sided limits exist and are equal) $\Rightarrow f$ is differentiable at $x=2.$ $f(x)$ satisfies ($2$).
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