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: 61

Answer

$$ \lim _{x \rightarrow 1^{+}} x^{1 /(1-x)}=\frac{1}{e} $$

Work Step by Step

$$ \lim _{x \rightarrow 1^{+}} x^{1 /(1-x)} $$ Let $$ y= x^{1 /(1-x)} $$ Then $$ \ln y=\frac{1}{1-x} \ln x=\frac{\ln x}{1-x} $$ notice that as $ \quad x \rightarrow 1^{+} \quad $ we have $ \quad \ln x \rightarrow 0 \quad $ and $ \quad 1-x \rightarrow 0 $ so l’Hospital’s Rule gives $$ \begin{aligned} \lim _{x \rightarrow 1+} \ln y& =\lim _{x \rightarrow 1+} \frac{1}{1-x} \ln x \\ &=\lim _{x \rightarrow 1+} \frac{\ln x}{1-x} \quad\quad \rightarrow \frac{0}{0}\\ &\stackrel{\mathrm{H}}{=} \lim _{x \rightarrow 1+} \frac{1 / x}{-1} \\ &=-1 \end{aligned} $$ Hence, we have $$ \begin{aligned} \lim _{x \rightarrow 1^{+}} x^{1 /(1-x)} & =\lim _{x \rightarrow 1^{+}} e^{\ln y} \\ &=e^{-1} \\ &=\frac{1}{e} \end{aligned} $$
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