Elementary Differential Equations and Boundary Value Problems 9th Edition

Published by Wiley
ISBN 10: 0-47038-334-8
ISBN 13: 978-0-47038-334-6

Chapter 2 - First Order Differential Equations - 2.2 Separable Equations - Problems - Page 50: 37

Answer

The solution of the differential equation is given by $$ | x| ^{3} | x^{2}-5y^{2}| = c $$ where c is an arbitrary constant.

Work Step by Step

$$ y^{'}=\frac{dy}{dx}=\frac{x^{2}-3y^{2}}{2xy} \quad (*) $$ Divide each term in the numerator and denominator by the highest power of $x$ , namely $x^{2}$. We obtain $$ y^{'}=\frac{dy}{dx}=\frac{1-3(\frac{y}{x})^{2}}{2(\frac{y}{x})} =f(x,y) $$ where $f(x,y)$ can be expressed as a function of the ratio $\frac{y}{x}$ only . Thus eq. (*) is homogeneous. Now we assume a new dependent variable $ v = \frac{y}{x }$ and we can express $$ y^{'}=\frac{dy}{dx}= x \frac{dv}{dx}+v =\frac{ 1-3v^{2}}{2v} $$ $$ x \frac{dv}{dx} = \frac{ 1-3v^{2}}{2v} -v = \frac{ 1-5v^{2}}{2v} \quad (**) $$ The differential equation (**) is separable and can be written as $$ \frac{2vdv}{{1-5v^{2}}} = \frac{dx}{x} $$ integrating the left side with respect to $v$ and the right side with respect to $x$ gives $$ \int{ \frac{2vdv}{{1-5v^{2}}} }= \int{ \frac{dx}{x}} $$ we can obtain $v$ implicitly in terms of $ x$ as the follows $$ \frac{-1}{5}ln| 1-5v^{2} |= ln | c x | $$ where c is an arbitrary constant. $$ | 1-5v^{2}| = c | x| ^{-5} $$ we find the solution of Eq. (*) by replacing $v$ by $ y/x $ in the previous expression, $$ | x| ^{5} | 1-5(\frac{y}{x})^{2}| = c $$ i.e. $$ | x| ^{3} | x^{2}-5y^{2}| = c $$ where c is an arbitrary constant.
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