Let u = f (x, z) and Z (x, y) be a function determined by the equation z = x YP (z)

Let u = f (x, z) and Z (x, y) be a function determined by the equation z = x YP (z)

DZ = D [XYP (z)] = YP (z) DX + XP (z) dy + XYP '(z) DZ, so DZ = [YP (z) DX + XP (z) dy] / [1 - XYP' (z)] Du = DF (x, z) = f'x (x, z) DX + f'z (x, z) DZ = f'x (x, z) DX + f'z (x, z) * {[YP (z) DX + XP (z) dy]} / [1 - XYP '(z)] = {f'x (x, z) + f'z (x, z) *