Thomas' Calculus 13th Edition

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

Chapter 7: Transcendental Functions - Section 7.5 - Indeterminate Forms and L'Hopital's Rule - Exercises 7.5 - Page 409: 39

Answer

$-\infty$

Work Step by Step

Here, we have $\lim\limits_{x \to 0} f(0)=\dfrac{\infty}{-\infty}$ This shows an indeterminate form of limit, thus we will apply L-Hospital's rule such as: $\lim\limits_{x \to \infty} f(x)=\lim\limits_{x \to \infty} \dfrac{p'(x)}{q'(x)}$ $ \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 $\lim\limits_{x \to 0} \dfrac{2 (\ln x)}{\cos x}(\lim\limits_{x \to 0} \dfrac{ \sin x}{x})$ or, $\lim\limits_{x \to 0} \dfrac{2 (\ln x)}{\cos x}(1)=\dfrac{2 (-\infty)}{1}=-\infty$
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