Calculus: Early Transcendentals 8th Edition

Published by Cengage Learning
ISBN 10: 1285741552
ISBN 13: 978-1-28574-155-0

Chapter 4 - Section 4.4 - Indeterminate Forms and l''Hospital''s Rule - 4.4 Exercises - Page 312: 69

Answer

$\lim\limits_{x \to \infty}(1+\frac{2}{x})^{x}=e^{2}=7.389$

Work Step by Step

We can look at a graph of $f(x)=(1+\frac{2}{x})^{x}$ and see that as $x$ gets larger and larger, $f(x)$ gets closer to 7.389. $\lim\limits_{x \to \infty}(1+\frac{2}{x})^{x}=(1+\frac{2}{\infty})^{\infty}=1^{\infty}$ We have an indeterminate form of $1^{\infty}$, we can rewrite this in terms of natural logs. $\ln((1+\frac{2}{x})^{x})=\ln y=x\ln(1+\frac{2}{x})$ $\lim\limits_{x \to \infty} x\ln(1+\frac{2}{x})=\infty*\ln(1+\frac{2}{\infty})=\infty*\ln1=\infty*0$ We now have an indeterminate form of $\infty*0$. We can rewrite this into $\lim\limits_{x \to \infty} \frac{ln(1+\frac{2}{x})}{\frac{1}{x}}$ and apply L'Hospital's rule. $\lim\limits_{x \to \infty} \frac{ln(1+\frac{2}{x})}{\frac{1}{x}}=\lim\limits_{x \to \infty} \frac{-2x^{2}}{-x^{2}-2x}$ $\lim\limits_{x \to \infty} \frac{-2x^{2}}{-x^{2}-2x}=\lim\limits_{x \to \infty} \frac{-4x}{-2x-2}$ $\lim\limits_{x \to \infty} \frac{-4x}{-2x-2}=\lim\limits_{x \to \infty} \frac{-4}{-2}$ $\lim\limits_{x \to \infty} \frac{-4}{-2}=\lim\limits_{x \to \infty} 2$ Remember, that this is in terms of logs, in order to cancel out the logs by raising $e$ to the power of the answer $\lim\limits_{x \to \infty} 2=\lim\limits_{x \to \infty}\ln y= \lim\limits_{x \to \infty}e^{ln y}=\lim\limits_{x \to \infty} e^{2}$ $e^{2}=7.389$
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