Calculus 8th Edition

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

Chapter 7 - Techniques of Integration - 7.8 Improper Integrals - 7.8 Exercises - Page 574: 31

Answer

Divergent

Work Step by Step

Let \[I=\int_{-2}^{3}\:\frac{1}{x^4}\,dx\] Since 0 is point of infinite discontinuity of integrand $\frac{1}{x^4}$ and $-2<0<3$ \[I=\int_{-2}^{0}\:\frac{1}{x^4}\,dx+\int_{0}^{3}\:\frac{1}{x^4}\,dx\;\;\;\ldots (1)\] Let \[I_1=\int\frac{1}{x^4}dx=\int x^{-4}dx\] \[I_1=\frac{-1}{3x^3}\;\;\;\ldots (2)\] Let \[I_2=\int_{-2}^{0}\:\frac{1}{x^4}\,dx\] \[I_2=\lim_{t_1\rightarrow 0^-}\int_{-2}^{t}\:\frac{1}{x^4}\,dx\;\;\;\ldots (3)\] Using (2) in (3) \[I_2=\lim_{t_1\rightarrow 0^-}\left[\frac{-1}{3x^3}\right]_{-2}^t\] \[I_2=\lim_{t_1\rightarrow 0^-}\left[\frac{-1}{3t^3}+\frac{1}{3(-8)}\right]\] $\;\;\;\;\;\;\;\;\;\;\;\Rightarrow $ does not exist Since limit on R.H.S. of (3) does not exist so $I_2$ is divergent From (1) Consequently, $I$ is divergent.
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