Suppose that lines l1, l2, l3 are lines in an affine plane. Suppose that lines l1, l2 are parallel. Prove that if l1, l3 intersect then l2, l3 intersect. Solution Here we have the rule that when two parallel lines are intersected by a transversal, then the exteriior corresponding angles, thus formed are equal to each other. Now it is given that l1 and l2 are parallel lines and l3 intersects l1, that means l3 is transversal here, so that the exterior angles thus forming on line l1 and l3 will be equal to each other. Now as both exterior angles are forming at two different intersecting points of parallel lines l1 and l2, when intersected by line l3, that proved that being parallel lines, line l3 intersects both lines l1 and l2. Proved.