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 - 1.9 Exact Differential Equations - Problems - Page 92: 6

Answer

\[y(x)=\sqrt{\frac{C+x^2}{x}}\]

Work Step by Step

$(y^2-2x)dx+2xydy=0$ ___(1) Here, $M(x,y)=y^2-2x$ $N(x,y)=2xy$ $M_y=2y\;\;\;\;,\;\;\;\;N_x=2y$ $M_y=N_x$ $\Rightarrow$ (1) is exact differential equation Therefore there exists a potential function $\phi$ such that $\frac{\partial\phi}{\partial x}=y^2-2x$ ___(2) $\frac{\partial\phi}{\partial y}=2xy$ ___(3) Integrating (2) with respect to $x$ holding $y$ fixed $\phi =y^2x-x^2+h(y)$ ___(4) Where $h(y)$ is arbitrary function of $y$ Differentiate (4) with respect to $y$ holding $x$ fixed $\frac{\partial\phi}{\partial y}=2xy+\frac{dh}{dy}$ ___(5) From (4) and (5) $\frac{dh}{dy}=0 \Rightarrow h(y)=C_1$ $C_1$ is constant From (4) $\phi(x,y)=y^2x-x^2+C_1$ One parameter family is solutions is given by $\phi(x,y)=C_2$ $\Rightarrow y(x)=\sqrt{\frac{C+x^2}{x}}$ Where, $C=C_2-C_1$. Hence general solution of (1) is $y(x)=\sqrt{\frac{C+x^2}{x}}$.
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