Calculus (3rd Edition)

Published by W. H. Freeman
ISBN 10: 1464125260
ISBN 13: 978-1-46412-526-3

Chapter 7 - Exponential Functions - 7.7 L'Hôpital's Rule - Exercises - Page 367: 43

Answer

$0$

Work Step by Step

We have $$ \lim _{t \rightarrow 0^+}(\sin t)(\ln t)=\lim _{t \rightarrow 0^+}\frac{\ln t}{\csc t}=\frac{\infty}{\infty} $$ which is an intermediate form, so we can apply L’Hôpital’s Rule as follows $$ \lim _{t \rightarrow 0^+}\frac{\ln t}{\csc t}= \lim _{t \rightarrow 0^+}\frac{1/ t}{-\csc t\cot t}\\ = \lim _{t \rightarrow 0^+}\frac{-\sin^2t}{t\cos t}=\frac{0}{0} $$ which is an intermediate form, so we can apply L’Hôpital’s Rule as follows $$\lim _{t \rightarrow 0^+}\frac{-\sin^2t}{t\cos t}= \lim _{t \rightarrow 0^+}\frac{-2\sin t\cos t}{\cos t+-t\sin t}=\frac{0}{1+0}=0. $$
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