Question

Find Integrating Fector: ylogy dx + ( x-logy )dy=0

Answer 1

Rewrite the ODE as

$y\log y\,\mathrm dx+(x-\log y)\,\mathrm dy=0\iff y\log y\dfrac{\mathrm dx}{\mathrm dy}+x=\log y$
$\dfrac{\mathrm dx}{\mathrm dy}=\dfrac1{y\log y}x=\dfrac1y$

so that it is now linear in $x$. An integrating factor would

$\mu(y)=\exp\left(\displaystyle\int\frac{\mathrm dy}{y\log y}\right)=e^{\log(\log y)}=\log y$

Multiply both sides by $\mu(y)$ to get

$\log y\dfrac{\mathrm dx}{\mathrm dy}=\dfrac1yx=\dfrac{\log y}y$
$\dfrac{\mathrm d}{\mathrm dy}[x\log y]=\dfrac{\log y}y$
$x\log y=\displaystyle\int\frac{\log y}y\,\mathrm dy$
$x\log y=\dfrac12\log^2y+C$
$x=\dfrac12\log y+\dfrac C{\log y}$