20. Determine the angle between a = (5, 3, 0) and b = ( -1, 0, 8) [3 Marks] 21. The vertices of a quadrilateral are A = [0, 3, 5], B = [-3, -1, 17], C = [4, -3, 15] & D = [7, 1,3]. Prove that ABCD is a parallelogram. [5 Marks]