hello, i have task Z= y*cos*(x-y) in point T (π; π/2; ?) .....is this derivation all right?

Z´x = y*sin*(x-y)/T1= - π/2*sin*( π- π/2) = - π/2

Z´y = y*sin*(x-y) + cos*(x-y)/T1 = π/2 * sin*( π – π/2) + cos*( π – π/2) = π/2

Z0 = π/2*cos*(2 π – π) = π/2*cos* π/2 = π/2

2

Could anyone help me please?