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: 19

Answer

Convergent $\;,\;$ $\Large\frac{-1}{4}$

Work Step by Step

Let \[I=\int_{-\infty}^{0}ze^{2z}dz\] \[I=\lim_{t\rightarrow -\infty}\int_{t}^{0}ze^{2z}dz\;\;\;\ldots (1)\] Let \[I_1=\int ze^{2z}dz\] Using integration by parts \[I_1=z\int e^{2z}dz-\int \left((z)'\int e^{2z}dz\right)dz\] \[I_1=\frac{ze^{2z}}{2}-\frac{1}{2}\int e^{2z}dz\] \[I_1=\frac{ze^{2z}}{2}-\frac{e^{2z}}{4}\;\;\;\ldots (2)\] Using (2) in (1) \[I=\lim_{t\rightarrow -\infty}\left[\frac{ze^{2z}}{2}-\frac{e^{2z}}{4}\right]_{t}^{0}\] \[I=\lim_{t\rightarrow -\infty}\left[\left(0-\frac{1}{4}\right)-\left(\frac{te^{2t}}{2}-\frac{e^{2t}}{4}\right)\right]\] \[I=\frac{-1}{4}\] Since limit on R.H.S. of (1) exists so $I$ is convergent \[I=\frac{-1}{4}.\]
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