Thomas' Calculus 13th Edition

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

Chapter 3: Derivatives - Section 3.2 - The Derivative as a Function - Exercises 3.2 - Page 118: 68

Answer

$-\frac{2}{9}$

Work Step by Step

We have $\lim _{x\rightarrow -1}\frac {x^{2/9}-1}{x+1}$. Similarly, let's rewrite the expression as $\lim _{x\rightarrow -1}\frac {f(x)-f(-1)}{x-(-1)}$, where $f(x) = x^{2/9}$. Again, this is the definition of the derivative of $f(x)$ at $x=-1$, so we have: $\lim _{x\rightarrow -1}\frac {x^{2/9}-1}{x+1} = f'(-1)$. Taking the derivative of $f(x) = x^{2/9}$, we get $f'(x) = \frac{2}{9}x^{-7/9}$. Evaluating $f'(-1)$ gives us $\frac{2}{9}(-1)^{-7/9} = -\frac{2}{9}$. Therefore, the limit is $-\frac{2}{9}$.
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