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 534: 19

Answer

Convergent.

Work Step by Step

$$ \int_{-\infty}^{0} z e^{2 z} d z $$ Observe that the given integral is improper $$ \begin{aligned} \int_{-\infty}^{0} z e^{2 z} d z &=\lim _{t \rightarrow-\infty} \int_{t}^{0} z e^{2 z} d z \\ & \quad\quad\quad\left[\text { use integration by parts} \right] \\ & \quad\quad\quad\left[\begin{array}{c}{u=z, \quad d v=e^{2z} dz } \\ {d u=dz, \quad v=\frac{1}{2}e^{2z} \quad}\end{array}\right] \\ &= \lim _{t \rightarrow-\infty}\left[ \frac{1}{2} z e^{2 z} -\frac{1}{2}\int_{t}^{0} e^{2 z} d z \right]\\ &=\lim _{t \rightarrow-\infty}\left[\frac{1}{2} z e^{2 z}-\frac{1}{4} e^{2 z}\right]_{t}^{0} \\ &=\lim _{t \rightarrow-\infty}\left[\left(0-\frac{1}{4}\right)-\left(\frac{1}{2} t e^{2 t}-\frac{1}{4} e^{2 t}\right)\right] \\ &=\lim _{t \rightarrow-\infty}\left[\left(0-\frac{1}{4}\right)-\left(\frac{1}{2}\frac{t}{e^{-2 t}} -\frac{1}{4} e^{2 t}\right)\right] \\ & \quad\quad\quad\quad[\text { by L'Hospital's Rule }] \\ &=-\frac{1}{4}-0+0 \\ &=-\frac{1}{4} . \end{aligned} $$ It follows that the integral $$ \int_{-\infty}^{0} z e^{2 z} d z $$ is convergent.
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