Let P be the intersection of lines l1 and l2. Let S1 and S2 be two
circles externally tangent at P and both tangent to l1, and let T1
and T2 be two circles externally tangent at P and both tangent to l2.
Let A be the second intersection of S1 and T1,B that of S1 and T2,C that of S2 and T1, and D that of S2 and T2. Show that the points A,B,C,D are concyclic if and only if l1 and l2 are perpendicular. geometryrectangleparallelogramgeometry unsolved