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: 11

Answer

$f(x)$ does not satisfy the hypotheses ($f$ is not differentiable at $x=-1$)

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 $x=-1.$ $\displaystyle \lim_{x\rightarrow-1^{-}}f(x)=\lim_{x\rightarrow-1^{-}}(x^{2}-x)=1+1=2$ $\displaystyle \lim_{x\rightarrow-1^{+}}f(x)=\lim_{x\rightarrow-1^{+}}(2x^{2}-3x-3)=2+3-3=2$ So $\displaystyle \lim_{x\rightarrow-1}f(x)$ exists and equals $f(-1)=2$. f is continuous at $x=-1 \Rightarrow$ hypothesis (1) is satisfied. $(2) $ Observe what happens with the derivative at $x=-1.$ $\displaystyle \left.\frac{d}{dx}(x^{2}-x)\right|_{x=-1}=(\left.2x-1)\right|_{x=-1}=-3$ $\displaystyle \left.\frac{d}{dx}(2x^{2}-3x-3)\right|_{x=-1}=\left. (4x-3)\right|_{x=-1}=-7$ This means that $\displaystyle \lim_{h\rightarrow 0}\frac{f(x+h)-f(-1)}{h}$ does not exist (the one-sided limits are different) $\Rightarrow f$ is not differentiable at $x=-1.$ $f(x)$ does not satisfy (2).
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