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 ray + y dy dr. -1 /2 (Make sure you explain briefly how you arrived at your answer.) [2+1=3 marks]