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 - 8.8 A Differential Equation with Nonconstant Coefficients - Problems - Page 567: 17

Answer

See below

Work Step by Step

Given $x^2y''-xy'+5y=8x(\ln x)^2$ In this case the substitution $y(x) = x^r$ yields the indicial equation $$r(r-1)-r+5=0\\ r^2-2r+5=0$$ It follows that two linearly independent solutions to the given differential equation are $u_1(x)=\cos (2 \ln x)\\ u_2(x)=\sin (2\ln x)$ so that the general solution is $y(x)=u_1(x)\cos (2\ln x)+u_2(x)\sin (2\ln x)$ where $u_1$ and $u_2$ are determined from $u_1'(x)\cos (2\ln x)+u_2'(x)\sin (2\ln x)=0\\ u_1'(x)(\cos (2\ln x))'+u'_2(x)(\sin (2\ln x))'=8x(\ln x)^2$ Hence, $u_1'=-\frac{4\sin (2\ln x)}{x}(\ln x)^2\\ u_2'=\frac{4\cos (2\ln x)}{x}(\ln x)^2$ which upon integration gives $u_1(x)=2(\ln x)^2\cos (2\ln x)-2\ln \sin(2\ln x)-\cos (2\ln x)\\ u_2(x)=2(\ln x)^2\sin (2\ln x)-2\ln \cos(2\ln x)+2\sin (2\ln x)$ Thus, the given equation has general solution $y(x)=c_1\cos (2\ln x)+c_2\sin (2\ln x)+2(\ln x)^2x$
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