Differential Equations and Linear Algebra (4th Edition)

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

Chapter 1 - First-Order Differential Equations - 1.12 Chapter Review - Additional Problems - Page 110: 40

Answer

See below

Work Step by Step

Given: $$y'+x^2y=x^3y^3\\\frac{dy}{dx}+x^2y=x^3y^3$$ This is a Bernoulli equation where $p(x)=x^2\\q(x)=x^3\\n=3$ Divide both sides by $y^3$ we have: $y^{-3}\frac{dy}{dx}+x^2y^{-2}=x^3$ Substitute $u=y^{-2} \rightarrow \frac{du}{dx}=-2y^{-3}\frac{dy}{dx}\\ \rightarrow -\frac{1}{2}\frac{du}{dx}+x^2u=x^3\\ \rightarrow \frac{du}{dx}-2x^2u=-2x^3$ Integrating factor for this will be: $I(x)=e^{-2x^2dx}=e^{-\frac{2}{3}x^3}$ The equation becomes: $\frac{d}{dx}(ue^{-\frac{2}{3}x^3})=-2x^3e^{-\frac{2}{3}x^3}$ Integrate both sides: $ue^{-\frac{2}{3}x^3}=\int-2x^3e^{-\frac{2}{3}x^3}dx$ Substitute back $u=y^{-2}$ we have: $y^{-2}=e^{\frac{2}{3}x^3}\int -2x^3e^{-\frac{2}{3}x^3}dx$ The solution to this is, $y(x)=\pm \frac{1}{\sqrt e^{\frac{2}{3}x^3}\int -2x^3e^{-\frac{2}{3}x^3}dx}$
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