Calculus: Early Transcendentals 8th Edition

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

Chapter 7 - Section 7.8 - Improper Integrals - 7.8 Exercises - Page 535: 55

Answer

$$ \int_{0}^{\infty} \frac{d x}{\sqrt{x}(1+x)} $$ The given improper integral is convergent and its value is $$ \int_{0}^{\infty} \frac{d x}{\sqrt{x}(1+x)}=\pi $$

Work Step by Step

$$ \int_{0}^{\infty} \frac{d x}{\sqrt{x}(1+x)} $$ The given improper integral can be expressed as a sum of improper integrals of Type 2 and Type 1 as follows: $$ \int_{0}^{\infty} \frac{d x}{\sqrt{x}(1+x)}=\int_{0}^{1} \frac{d x}{\sqrt{x}(1+x)}+\int_{1}^{\infty} \frac{d x}{\sqrt{x}(1+x)} $$ Observe that the first integral is improper because $ f(x)= \frac{1}{\sqrt{x}(1+x)} $ has the vertical asymptote $ x =0 $. Since the infinite discontinuity occurs at the left endpoint of $ [0,1] $ $$ \begin{split} \int_{0}^{\infty} \frac{d x}{\sqrt{x}(1+x)} &=\int_{0}^{1} \frac{d x}{\sqrt{x}(1+x)}+\int_{1}^{\infty} \frac{d x}{\sqrt{x}(1+x)} \\ &=\lim _{t \rightarrow 0^{+}} \int_{t}^{1} \frac{d x}{\sqrt{x}(1+x)}+\lim _{t \rightarrow \infty} \int_{1}^{t} \frac{d x}{\sqrt{x}(1+x)} \quad (1) \end{split} $$ Now $$ \begin{split} \int \frac{d x}{\sqrt{x}(1+x)} &=\int \frac{d x}{\sqrt{x}(1+x)} \\ & \quad\quad\left[\begin{array}{c}{u=\sqrt{x}, x=u^{2}} \\ {d x=2 u d u}\end{array}\right] \\ & =\int \frac{2 u d u}{u\left(1+u^{2}\right)} \\ & =2 \int \frac{d u}{1+u^{2}} \\ &=2 \tan ^{-1} u+C \\ &=2 \tan ^{-1} \sqrt{x}+C \end{split} $$ substituting in Eq. (1 ) we obtain that, $$ \begin{split} \int_{0}^{\infty} \frac{d x}{\sqrt{x}(1+x)} &=\lim _{t \rightarrow 0^{+}} \left[ 2 \tan ^{-1} \sqrt{x} \right]_{t}^{1} +\lim _{t \rightarrow \infty}\left[ 2 \tan ^{-1} \sqrt{x} \right]_{1}^{t} \\ &=\lim _{t \rightarrow 0+}\left[2\left(\frac{\pi}{4}\right)-2 \tan ^{-1} \sqrt{t}\right] +\\ &\quad+\lim _{t \rightarrow \infty}\left[2 \tan ^{-1} \sqrt{t}-2\left(\frac{\pi}{4}\right)\right] \\ &=\frac{\pi}{2}-0+2\left(\frac{\pi}{2}\right)-\frac{\pi}{2}\\ &=\pi \end{split} $$ The given improper integral is convergent and its value is $$ \int_{0}^{\infty} \frac{d x}{\sqrt{x}(1+x)}=\pi $$
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