Find the differential of f(x,y)=z=xcos(xy)
I think you're looking for dz given that z is a function of x and y, so dz will be in terms of dx and dy. It's just the chain rule for two variables in a slightly different form:
$\displaystyle dz=\frac{\partial{z}}{\partial{x}}dx$$\displaystyle +\frac{\partial{z}}{\partial{y}}dy$
Is that what you were looking for?
- Hollywood