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: 9

Answer

$$\rho = \infty$$

Work Step by Step

1. Find the Taylor series: $$sin(x) = sin(0) + cos(0)x - \frac{sin(0)}{2!}x^2 - \frac{cos(0)}{3!}x^3 + \frac{sin(0)x^4}{4!} + \frac{cos(0)x^5}{5!}...$$ $$sin(x) = 0 + x - 0 - \frac{x^3}{3!} + 0 + \frac{x^5}{5!}...$$ $$sin(x) = \sum^{\infty}_{n = 0} (-1)^n \frac{x^{2n+1}}{(2n+1)!}$$ 2. Use the ratio test to test for convergence: $$\lim_{n \longrightarrow \infty} \Bigg| \frac{ (-1)^{n+1} \frac{x^{2(n+1)+1}}{(2(n+1)+1)!} } { (-1)^{n} \frac{x^{2n+1}}{(2n+1)!} } \Bigg | = \lim_{n \longrightarrow \infty} \Bigg| (-1) \frac{x^2}{(2n+2)(2n+3)} \Bigg| = 0$$ - Therefore, the series converges absolutely when $0 \lt 1$, which is true for any $x$ value. $$\rho = \infty$$
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