Originally Posted by
Jhevon linear, non-linear, makes little difference. you want to find $\displaystyle \frac {\partial ^2 f}{\partial x ^2}$ and $\displaystyle \frac {\partial ^2 f}{\partial y^2}$ for $\displaystyle f(x,y) = \frac {xy}{x - y}$
to find the first, differentiate with respect to x twice. that is, treat y as a constant. so you would just differentiate normally thinking of y as just any other number, like 2 or something.
a similar approach can get you $\displaystyle \frac {\partial ^2 f}{\partial y^2}$
can you start?