Calculus, 10th Edition (Anton)

Published by Wiley
ISBN 10: 0-47064-772-8
ISBN 13: 978-0-47064-772-1

Chapter 2 - The Derivative - Chapter 2 Review Exercises - Page 185: 24

Answer

$f(2)=3$ $f'(2)=-1$

Work Step by Step

We have: $\frac{\lim\limits_{x \to 2}x^{3}f(x)-24}{\lim\limits_{x \to 2}x-2}=28$ $\because$ The denominator's limit is 0, and $f$ is differentiable at $x = 2$, the numerator's limit must also be $0$. $\implies \lim\limits_{x \to 2}x^{3}f(x)-24=0 \implies2^{3}f(2)=24 \implies f(2)=3$ Finding $f'(2)$: Differentiating both numerator and denominator, ($\because$ According to L'Hopital's rule, $\lim\limits_{x\to c}\frac{f(x)}{g(x)} = \lim\limits_{x\to c}\frac{f'(x)}{g'(x)}$) $\implies \lim\limits_{x\to2}\frac{x^{3}f'(x)+3x^{2}f(x)}{1}=28$ Substituting $x=2$, $\implies 8f'(2)+12f(2)=28. $ But, $f(2)=3$ (check above) $\implies 8f'(2)=28-36=-8 \implies f'(2)=-1$
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