In: Advanced Math
Let l and m be two lines in ordinary Euclidean geometry intersecting at point O. Let A, B and C be three distinct points on l and A', B' and C' three distinct points on m (none of them equal to O). Suppose that AB' is parallel to BA', and AC' is parallel to CA'. Prove, using Pappus' theorem in the Extended Euclidean Plane, that BC' and CB' are parallel.