Calculus: Early Transcendentals 9th Edition

Published by Cengage Learning
ISBN 10: 1337613924
ISBN 13: 978-1-33761-392-7

Chapter 7 - Section 7.8 - Improper Integrals - 7.8 Exercises - Page 550: 18

Answer

Diverges

Work Step by Step

$$\eqalign{ & \int_{ - \infty }^{ - 1} {\frac{{{x^2} + x}}{{{x^3}}}} dx \cr & {\text{Distribute the numerator}} \cr & = \int_{ - \infty }^{ - 1} {\left( {\frac{{{x^2}}}{{{x^3}}} + \frac{x}{{{x^3}}}} \right)} dx \cr & = \int_{ - \infty }^{ - 1} {\left( {\frac{1}{x} + \frac{1}{{{x^2}}}} \right)} dx \cr & {\text{Using the definition of improper integrals }} \cr & \underbrace {\int_{ - \infty }^b {f\left( x \right)dx = \mathop {\lim }\limits_{t \to - \infty } } \int_t^b {f\left( x \right)} dx}_ \Downarrow \cr & \int_{ - \infty }^{ - 1} {\left( {\frac{1}{x} + \frac{1}{{{x^2}}}} \right)} dx = \mathop {\lim }\limits_{t \to - \infty } \int_t^{ - 1} {\left( {\frac{1}{x} + \frac{1}{{{x^2}}}} \right)} dx \cr & {\text{Integrate}} \cr & = \mathop {\lim }\limits_{t \to - \infty } \left[ {\ln \left| x \right| - \frac{1}{x}} \right]_t^{ - 1} \cr & = \mathop {\lim }\limits_{t \to - \infty } \left[ {\left( {\ln \left| { - 1} \right| - \frac{1}{{ - 1}}} \right) - \left( {\ln \left| t \right| - \frac{1}{t}} \right)} \right] \cr & = \mathop {\lim }\limits_{t \to - \infty } \left[ {1 - \left( {\ln \left| t \right| - \frac{1}{t}} \right)} \right] \cr & {\text{Evaluate the limit when }}t \to - \infty \cr & = 1 - \infty - 0 \cr & = - \infty \cr & {\text{Therefore}}{\text{, the integral diverges.}} \cr} $$
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