Find the equation for the plane tangent to each surface
z = f(x, y) at the indicated point.
(a) z = x3 + y3 − 5xy, at the point (1, 2, 1)
(b) z = (cos(x))(cos(y)), at the point (0,π/2,0)
(c) z = (cos(x))(sin(y)), at the point (0,π/2,1)