ABCD is a cyclic quadrilateral inscribed in circle O. Let O1 be the A-excenter of ABC and O2 the A-excenter of ABD. Show that A,B,O1,O2 is concyclic, and O passes through the center of (ABO1O2). Recall that for concyclic X,Y,Z,W, the notation (XYZW) denotes the circumcircle of XYZW. geometryrectangleexcirclecyclic quadrilateral