(a) Let f(x, y) = 23 +3 -2x y". Calculate fx, fy, fax, fyy and fry. Hence show that (3/4, 3/4) is a stationary point and determine the nature of this stationary point. (b) Let f(x, y) = ax2 + backy + cy + d with a, b, c, d * 0. By calculating fx, fy, fax, fyy and fxy, Show that f (x, y) has a stationary point at (0, 0) and determine the conditions (in terms of a, b, c and d) under which this point is a maximum