Elementary Differential Equations and Boundary Value Problems 9th Edition

Published by Wiley
ISBN 10: 0-47038-334-8
ISBN 13: 978-0-47038-334-6

Chapter 5 - Series Solutions of Second Order Linear Equations - 5.1 Review of Power Series - Problems - Page 249: 10

Answer

$$\rho = \infty$$

Work Step by Step

1. Find the Taylor series: $$e^x = e^0 + e^0x + \frac{e^0}{2!}x^2 + \frac{e^0}{3!}x^3... $$ $$ e^x = 1 + x + \frac{x^2}{2!} + \frac{x^3}{3!}... $$ $$ e^x = \sum^{\infty}_{n = 0} \frac{x^n}{n!} $$ 2. Use the ratio test to test for convergence: $$\lim_{n \longrightarrow \infty} \Bigg| \frac{ \frac{ x^{n+1} }{(n+1)!} } { \frac{x^n}{n!} } \Bigg | = \lim_{n \longrightarrow \infty} \Bigg| \frac{x}{n+1} \Bigg| = 0$$ - Therefore, the series converges absolutely when $0 \lt 1$, which is true for any $x$ value. $$\rho = \infty$$
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