Calculus 8th Edition

Published by Cengage
ISBN 10: 1285740629
ISBN 13: 978-1-28574-062-1

Chapter 6 - Inverse Functions - 6.8 Indeterminate Forms and I'Hospital's Rule - 6.8 Exercises - Page 500: 51

Answer

$$ \lim _{x \rightarrow 1}\left(\frac{x}{x-1}-\frac{1}{\ln x}\right) =\frac{1}{2} $$

Work Step by Step

$$ \lim _{x \rightarrow 1}\left(\frac{x}{x-1}-\frac{1}{\ln x}\right) $$ First notice that $$\lim _{x \rightarrow 1} \frac{x}{x-1} \rightarrow \infty $$ and $$\lim _{x \rightarrow 1} \frac{1}{\ln x} \rightarrow \infty ,$$ so the limit is indeterminate. Here we use a common denominator: $$\begin{aligned} \lim _{x \rightarrow 1}\left(\frac{x}{x-1}-\frac{1}{\ln x}\right) &=\lim _{x \rightarrow 1} \frac{x \ln x-(x-1)}{(x-1) \ln x} \rightarrow \frac{0}{0} \\ &\stackrel{\text { H }}{=} \lim _{x \rightarrow 1} \frac{x(1 / x)+\ln x-1}{(x-1)(1 / x)+\ln x} \\ &=\lim _{x \rightarrow 1} \frac{\ln x}{1-(1 / x)+\ln x} \\ &=\lim _{x \rightarrow 1} \frac{1 / x}{1 / x^{2}+1 / x} \cdot \frac{x^{2}}{x^{2}} \\ &=\lim _{x \rightarrow 1} \frac{x}{1+x} \\ &=\frac{1}{1+1}\\ &=\frac{1}{2}. \end{aligned} $$ Note that the use of l’Hospital’s Rule is justified because $ x \ln x-(x-1) \rightarrow 0 $ and $(x-1) \ln x \rightarrow 0 $ as$ \rightarrow 1$
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