Let A(0,0,0),B(1,1,1),C(3,2,1) and D(3,1,2) be four points. The angle between the planes through the points A,B,C and through the points A,B,D is
If the co-ordinates of the points A, B, C, D be (1, 2, 3), (4, 5, 7), (-4, 3, -6) and (2, 9, 2) respectively, then the angle between the lines AB and CD is