# Math Help - how to pick a variable here??

1. ## how to pick a variable here??

$
xyy'=\frac{x^2}{cos(\frac{y}{x})}+y^2
$

i know i need to look for a variable which both its derivative and him selft are present in the equation.
i tried t=y/x
but its derivative is not present in the equation

??

2. This appears to be homogeneous if you divide all thru by xy.

Then you'll have a function in y/x. Make the substitution z = y/x, substitute for dy/dx (it's z + x dz/dx) and you will find you have a diffeq with separable variables.