Differentiate multivariable function with respect to another function

Can you please help me with the following problem:

Given a function of the form f(x,y) = (a-b x)^2 - (c - d y)^2, how can I tell how does f change when (x-y) change?

Re: Differentiate multivariable function with respect to another function

Hey Essa.

Do you know about directional derivatives?

Re: Differentiate multivariable function with respect to another function

I'm a little weak on multi-variable calculus, but if g=x-y, can't you just do:

$\displaystyle \frac{df}{dg} = \frac{df/dx}{dg/dx} + \frac{df/dy}{dg/dy}$

- Hollywood