4. (a) Evaluate xy3 - x2 (y + 1) dy dx. (Make sure you show all your working). (b) Describe precisely (i.e. in the form [a, b] x [c, d] for appropriate a, b, c, d E R) the region of integration used in the integral x y + y dy dx. (Make sure you explain briefly how you arrived at your answer. ) [2+1=3 marks]