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 549: 9

Answer

Diverges

Work Step by Step

$$\eqalign{ & \int_{ - 2}^\infty {\frac{1}{{x + 4}}} dx \cr & {\text{Using the definition of improper integrals }} \cr & \underbrace {\int_a^\infty {f\left( x \right)dx = \mathop {\lim }\limits_{t \to \infty } } \int_a^t {f\left( x \right)} dx}_ \Downarrow \cr & \int_{ - 2}^\infty {\frac{1}{{x + 4}}} dx = \mathop {\lim }\limits_{t \to \infty } \int_{ - 2}^t {\frac{1}{{x + 4}}} dx \cr & {\text{Integrating}} \cr & = \mathop {\lim }\limits_{t \to \infty } \left[ {\ln \left| {x + 4} \right|} \right]_{ - 2}^t \cr & = \mathop {\lim }\limits_{t \to \infty } \left[ {\ln \left| {t + 4} \right| - \ln \left| { - 2 + 4} \right|} \right] \cr & = \mathop {\lim }\limits_{t \to \infty } \left[ {\ln \left| {t + 4} \right| - \ln \left| 2 \right|} \right] \cr & = \mathop {\lim }\limits_{t \to \infty } \left[ {\ln \left| {t + 4} \right|} \right] - \mathop {\lim }\limits_{t \to \infty } \left[ {\ln \left| 2 \right|} \right] \cr & {\text{Evaluate the limit when }}t \to \infty \cr & = \infty - \ln 2 \cr & = \infty \cr & {\text{Therefore}}{\text{, the integral diverges}} \cr} $$
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