Calculus: Early Transcendentals 8th Edition

Published by Cengage Learning
ISBN 10: 1285741552
ISBN 13: 978-1-28574-155-0

Chapter 11 - Section 11.4 - The Comparison Tests - 11.4 Exercises - Page 731: 24

Answer

$\Sigma$$\frac{n+3^{n}}{n+2^{n}}$ is divergent

Work Step by Step

Set $a_{n}$ = $\frac{n+3^{n}}{n+2^{n}}$ $b_{n}$ = $\frac{3^{n}}{2^{n}}$ = $\frac{3}{2}$^{n} $\lim\limits_{n \to \infty}$ $\frac{a_{n}}{b_{n}}$ = $\lim\limits_{n \to \infty}$ $\frac{\frac{n}{3^{n}}+1}{\frac{n}{2^{n}}+1}$ Then solve the two small limits respectively: $\lim\limits_{n \to \infty}$ $\frac{n}{3^{n}}$ = $\lim\limits_{n \to \infty}$ $\frac{1}{n \times 3^{n-1}}$ (L'Hopital rule) = 0 $\lim\limits_{n \to \infty}$ $\frac{n}{2^{n}}$ = $\lim\limits_{n \to \infty}$ $\frac{1}{n \times 2^{n-1}}$ (L'Hopital rule) = 0 Therefore $\lim\limits_{n \to \infty}$ $\frac{a_{n}}{b_{n}}$ = $\lim\limits_{n \to \infty}$ $\frac{\frac{n}{3^{n}}+1}{\frac{n}{2^{n}}+1}$ = $\frac{0+1}{0+1}$ = 1 > 0 since $\lim\limits_{n \to \infty}$ $\frac{a_{n}}{b_{n}}$ > 0 $\Sigma$$b_{n}$ is divergent ($\frac{3}{2}$ > 1 , $b_{n}$ is geometric series) then $\Sigma$$\frac{n+3^{n}}{n+2^{n}}$ is divergent by limit comparison test.
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