1. The problem statement, all variables and given/known data If z = f(x-y), show that dz/dx + dz/dy = 0 2. The attempt at a solution I thought: dz/dx = fx dz/dy = -fy which doesnt make sense really... because its not equal to 0. or maybe it should be: dz/dx = dz/df * df/dx = fx * ?? dz/dy = dz/df * df/dy = -fy * ??