f(x)=z=x+y/x-y
dz=fxdx+fydy=[[(x-y)-(x+y)]/(x-y)^2 ]dx+[[(x-y)+(x+y)]/(x-y)^2]dy
=-2y/(x-y)^2 dx+2x/(x-y)^2 dy
=2/(x-y)^2 (xdy-ydx)