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 - 8.10 Chapter Review - Additional Problems - Page 576: 63

Answer

See below

Work Step by Step

Given: $y''+4y'+4y=\frac{\ln x}{xe^{2x}}$ In this case the substitution $y(x) = x^r$ yields the indicial equation $r^2+4r+4=0$ Factor and solve the equation $(r+2)^2=0$ It follows that two linearly independent solutions to the given differential equation are $y_1(x)=e^{-2x}\\ y_2(x)=xe^{-2x}$ so that the particular solution is $y(x)=u_1e^{-2x}+u_2xe^{-2x}$ We first compute the appropriate derivatives $y'=u_1'(x)e^{-2x}+u_2'(x)xe^{-2x}\\ y''=u_1'(x)(e^{-2x})'+u_2'(x)(xe^{-2x})'$ Substituting these expressions into the given equation, we find that $u$ must satisfy $u_2'=\frac{\ln x}{x}\\ u_2=\frac{(\ln x)^2}{2}$ and $u_1'=-\ln x\\ u_1=-x\ln x+x$ Hence, the general solution is: $y(x)=e^{-2x}x-e^{-2x}x\ln x+\frac{1}{2}xe^{-2x}(\ln x)^2$
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