Question:
Consider the differential equation,
$y^{2} d x+\left(x-\frac{1}{y}\right) d y=0$. If value of $y$ is 1 when $x=1$, the the value of $x$ for which $y=2$, is :
Correct Option: , 4
Solution:
$y^{2} d x+x d y=\frac{d y}{y}$
$\frac{d x}{d y}+\frac{x}{y^{2}}=\frac{1}{y^{3}}$
$I F=e^{\int \frac{1}{y^{2}} d y}=e^{-\frac{1}{y}}$
$e^{-\frac{1}{y}} \cdot x=\int e^{-\frac{1}{y}} \cdot \frac{1}{y^{3}} d y+C$
$x e^{-\frac{1}{y}}=e^{-\frac{1}{y}}+\frac{e^{-\frac{1}{y}}}{y}+C$
$\mathrm{C}=-\frac{1}{\mathrm{e}}$
$x=\frac{3}{2}-\frac{1}{\sqrt{\mathrm{e}}}$ when $y=2$