1. The problem statement, all variables and given/known data If Z= F(x-y), show that Zx + Zy = 0 2. Relevant equations 3. The attempt at a solution Suppose I let Q = x-y. Then, by chain rule, Fx(Q) * 1 + Fy(Q) * -1. By identity, this statement must hold for all values x,y. In particular, it must hold for x=y. By x=y, Fy(Q) * 1 + Fy(Q) * -1 = 0. Is this legitimate?