Let ABCD a trapezium with AB parallel to CD,Ω the circumcircle of ABCD and A1,B1 points on segments AC and BC respectively, such that DA1B1C is a cyclic cuadrilateral. Let A2 and B2 the symmetric points of A1 and B1 with respect of the midpoint of AC and BC, respectively. Prove that points A,B,A2,B2 are concyclic. geometrytrapezoidcircumcirclecyclic quadrilateral