Find the equation for the plane tangent to eachsurface

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,?/2,0)

(c)z = (cos(x))(sin(y)), at thepoint (0,?/2,1)