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)