Differential Equations and Linear Algebra (4th Edition)

Published by Pearson
ISBN 10: 0-32196-467-5
ISBN 13: 978-0-32196-467-0

Chapter 8 - Linear Differential Equations of Order n - 8.9 Reduction of Order - Problems - Page 572: 11

Answer

$y(x)=C_1(1-2x) +C_2e^{2 x}+2x^2e^{2x}$

Work Step by Step

Given $xy''-(2x+1)y'+2y=8x^2e^{2x}$ with $y_1=e^{2x}$ We first compute the appropriate derivatives $y'=e^{2x}(u'+u)\\ y''=e^{2x}(u''+2u'-u)$ Substituting these expressions into the given equation, we find that $u$ must satisfy $xe^{2x}(u''+2u'-u)-(2x+1)e^{2x}(u'+u)+e^{2x}=8x^2e^{2x}$ which simplifies to $xu''+2xu'-u'=8x^2$ or equivalently $xw'+2x w-w=8x^2\\ xw'+(2x-1)u'=8x^2$ where $w = u′$ or equivalently, $w'-\frac{2x-1}{x}w=8x$ An integrating factor is $I=e^{\frac{2x-1}{x} dx}=2e^{2x-\log x}=\frac{e^{2x}}{x}$ So that $\frac{d}{dx}(\frac{e^{2x}}{x}w)=8x\frac{e^{2x}}{x}$ Integrating both sides with respect to x yields $\frac{e^{2x}}{x} w=4e^{2x}+C_1$ where $C_1$ is a constant. Thus $u'(x)=4x+C_1xe^{-2x}$ which can be integrated directly to obtain $u(x)=2x^2+C_1(1-2x)e^{-2x}+C_2$ The general solution is $y(x)=C_1(1-2x) +C_2e^{2 x}+2x^2e^{2x}$
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