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: 20

Answer

Convergent.

Work Step by Step

$$ \int_{2}^{\infty} y e^{-3 y} d y $$ Observe that the given integral is improper $$ \begin{aligned}\int_{2}^{\infty} y e^{-3 y} d y &=\lim _{t \rightarrow \infty} \int_{2}^{t} y e^{-3 y} d y \\ & \quad\quad\quad\left[\text { use integration by parts} \right] \\ & \quad\quad\quad\left[\begin{array}{c}{u=y, \quad d v=e^{-3y} dy } \\ {d u=dy, \quad v=\frac{-1}{3}e^{-3y} \quad}\end{array}\right] \\ &= \lim _{t \rightarrow \infty}\left[ \frac{-1}{3} y e^{-3 y} +\frac{1}{3}\int_{2}^{t} e^{-3 y} dy \right]\\ &=\lim _{t \rightarrow \infty}\left[-\frac{1}{3} y e^{-3 y}-\frac{1}{9} e^{-3 y}\right]_{2}^{t} \\ &=\lim _{t \rightarrow \infty}\left[\left(-\frac{1}{3} t e^{-3 t}-\frac{1}{9} e^{-3 t}\right)-\left(-\frac{2}{3} e^{-6}-\frac{1}{9} e^{-6}\right)\right] \\ &=\lim _{t \rightarrow \infty}\left[\left(-\frac{1}{3} \frac{t}{e^{3 t}} -\frac{1}{9} e^{-3 t}\right)-\left(-\frac{2}{3} e^{-6}-\frac{1}{9} e^{-6}\right)\right] \\ & \quad\quad\quad\quad[\text { by L'Hospital's Rule }] \\ &=0-0+\frac{7}{9} e^{-6}\\ &=\frac{7}{9} e^{-6} .\\ \end{aligned} $$ It follows that the integral $$ \int_{2}^{\infty} y e^{-3 y} d y $$ is convergent.
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