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: 33

Answer

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

Work Step by Step

$$ y^{'}=\frac{dy}{dx}=\frac{4y-3x}{2x-y} \quad (*) $$ Divide each term in the numerator and denominator by the highest power of $ x$ , namely $x$. We obtain $$ y^{'}=\frac{dy}{dx}=\frac{4(\frac{y}{x})-3}{2-(\frac{y}{x})} =f(x,y) $$ 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 $$ y^{'}=\frac{dy}{dx}= x \frac{dv}{dx}+v = \frac{4v-3}{2-v} $$ $$ x \frac{dv}{dx} = \frac{4v-3}{2-v}-v= \frac{v^{2}+2v-3}{2-v} \quad (**) $$ The differential equation (**) is separable and can be written as $$ [\frac{2-v}{{{v^{2}+2v-3}}}] dv= \frac{dx}{x} $$ by using partial fractions and some simplifications the provisos differential equation can be written as $$ [\frac{\frac{-5}{4}}{{v+3}}+\frac{\frac{1}{4}}{{v-1}}] dv= \frac{dx}{x} $$ integrating the left side with respect to $v$ and the right side with respect to $x$ gives $$ \frac{-5}{4} \int{\frac{dv}{v+3}}+\frac{1}{4} \int{\frac{dv}{v-1}}= \int{\frac{dx}{x}} $$ we obtain $$ \frac{-5}{4} ln | (v+3)|+\frac{1}{4} ln | (v-1)|= ln | x |+ ln | c | $$ where c is an arbitrary constant. simplification $$ ln | \frac{(v-1)}{(v+3)^{5} }|= 4 ln | xc | $$ we can obtain $v$ implicitly in terms of $ x$ as the follows $$ \frac{ |(v-1)|} {|(v+3)^{5} |}= | x |^{4}c $$ we find the solution of Eq. (*) by replacing $v$ by $ y/x $ in the previous expression. Hence the solution of eq. (*) is given by $$ \frac{ |(\frac{y}{x}-1)|} {|(\frac{y}{x}+3)^{5} |}= | x |^{4}c $$ this implies that $$ \frac{ |(\frac{y-x}{x})|} {|(\frac{y+3x}{x})^{5} |}= | x |^{4}c $$ Hence the solution of eq. (*) given by $$ | (y-x)|=| (y+3x)|^{5} c $$ where c is an arbitrary constant.
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