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 - 1.8 Change of Variables - Problems - Page 80: 50

Answer

$y=\left(\frac{\sec{x}+\tan{x}+C}{\tan{x}+\sec{x}}\right)^{-\frac{1+\sqrt{3}}{2}}$

Work Step by Step

Given $$(1-\sqrt{3})y'+y\sec{x}=y^{\sqrt{3}}\sec{x}$$ Note the equation is in Bernoulli form: $\frac{dy}{dx}+P(x)y=Q(x)y^n$ Now divide all terms by $y^n$: $$(1-\sqrt{3})y^{-\sqrt{3}}y'+y^{1-\sqrt{3}}\sec{x}=\sec{x}$$ Let $u=y^{1-\sqrt{3}}$. Thus, $\frac{1}{1-\sqrt{3}}\frac{du}{dx}=y^{-\sqrt{3}}\frac{dy}{dx}$ $$(1-\sqrt{3})\left(\frac{1}{1-\sqrt{3}}\right)\frac{du}{dx}+u\sec{x}=\sec{x}$$ $$\frac{du}{dx}+u\sec{x}=\sec{x}$$ Now solve as a first-order linear differential equation of the form $\frac{dy}{dx}+P(x)y=Q(x)$ First, find the integration factor $I(x)=e^{\int{P(x)}dx}$: $$I(x)=e^{\int{\sec{x}}dx}=e^{\ln{|\tan{x}+\sec{x}|}}=\tan{x}+\sec{x}$$ Multiply both sides by $I(x)$ and replace the left side with $\frac{d}{dx}[I(x)u]$: $$\frac{d}{dx}[(\tan{x}+\sec{x})u]=\sec{x}(\tan{x}+\sec{x})$$ $$\frac{d}{dx}[(\tan{x}+\sec{x})u]=\sec{x}\tan{x}+\sec^2{x}$$ Integrate both sides: $$\int{\frac{d}{dx}[(\tan{x}+\sec{x})u]}=\int{(\sec{x}\tan{x}+\sec^2{x})}dx$$ $$(\tan{x}+\sec{x})u=\sec{x}+\tan{x}+C$$ Solve for $y$: $$(\tan{x}+\sec{x})y^{1-\sqrt{3}}=\sec{x}+\tan{x}+C$$ $$y^{1-\sqrt{3}}=\frac{\sec{x}+\tan{x}+C}{\tan{x}+\sec{x}}$$ $$y=\left(\frac{\sec{x}+\tan{x}+C}{\tan{x}+\sec{x}}\right)^{-\frac{1+\sqrt{3}}{2}}$$
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