## Calculus: Early Transcendentals 8th Edition

Published by Cengage Learning

# Chapter 11 - Section 11.5 - Alternating Series - 11.5 Exercises - Page 736: 18

Divergent

#### Work Step by Step

Alternating series test: Suppose that we a have series $\Sigma a_n$, such that $a_{n}=(-1)^{n}b_n$ or $a_{n}=(-1)^{n+1}b_n$, where $b_n\geq 0$ for all $n$. Then if the following two conditions are satisfied, the series is convergent. 1. $\lim\limits_{n \to \infty}b_{n}=0$ 2. $b_{n}$ is a decreasing sequence. In the given problem, $b_{n}=cos\frac{\pi}{n}$ $\lim\limits_{n \to \infty}b_{n}=\lim\limits_{n \to \infty}cos\frac{\pi}{n}$ $=cos(0)$ $=1$ Thus, the limit is not zero. Hence, the given series is divergent by the divergence test.

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