University Calculus: Early Transcendentals (3rd Edition)

Published by Pearson
ISBN 10: 0321999584
ISBN 13: 978-0-32199-958-0

Chapter 4 - Section 4.5 - Indeterminate Forms and L'Hôpital's Rule - Exercises - Page 248: 39

Answer

$-\infty$

Work Step by Step

L'Hospital's rule states that $\lim\limits_{x \to \infty} f(x)=\lim\limits_{x \to \infty} \dfrac{A'(x)}{B'(x)}$ Here, $\lim\limits_{x \to 0} f(0)=\dfrac{\infty}{-\infty}$ This shows an indeterminate form of the limit, so we need to use L'Hospital's rule. $ \lim\limits_{x \to 0} \dfrac{\dfrac{2 \ln x}{x}}{\dfrac{\cos x}{\sin x}}=\lim\limits_{x \to 0} \dfrac{2 (\ln x) \sin x}{x \cos x}$ This implies that $\lim\limits_{x \to 0} \dfrac{2 (\ln x)}{\cos x}\lim\limits_{x \to 0} (\lim\limits_{x \to 0} \dfrac{2 (\ln x) \sin x}{x \cos x})=\lim\limits_{x \to 0} \dfrac{2 (\ln x)}{\cos x}(1)$ or, $\dfrac{2 (-\infty)}{\cos 0}=-\infty$
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