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: 31

Answer

$I(x,y)=x^1y^2$

Work Step by Step

We are given $2y(y+2x^2)dx+x(4y+3x^2)dy=0$ Multiplying equation (1) by $I(x)=x^ry^s$ $x^ry^s(2y(y+2x^2)dx+x(4y+3x^2)dy=0$ $x^ry^s(y+2x^2)dx+x^ry^sx(4y+3x^2)dy=0$ Here, $M(x,y)=x^ry^s(y^{-1}-x^{-1})$ $N(x,y)=x^ry^s(xy^{-2}-2y^{-1})$ $\frac{\partial M}{\partial y}=2x^ry^s((2(s+1)x^2+(s+2)y)$ $\frac{\partial N}{\partial x}=x^{r}y^s(3(r+3)x^2+4(r+1)y)$ The given diffirential equation is exact So $\frac{\partial N}{\partial x}=\frac{\partial M}{\partial y}$ $2x^ry^s((2(s+1)x^2+(s+2)y)=x^{r}y^s(3(r+3)x^2+4(r+1)y)$ $4(s+1)x^2+2(s+2)y=3(r+3)x^2+4(r+1)y$ $4(s+1)=3(r+3)$ $2s+4=4(r+1)$ $\rightarrow r=1, s=2$ Hence here, $I(x,y)=x^1y^2$
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