Cyclic quadrilateral and angle bisectors

The question won't post above, so here it is:

quadrilateral ABCD is inscribed in a circle where AB>DC and AB is not parallel to DC. If X is the intersection of their diagonals, let the perpindicular from X to AB be called Y where XY intersects AB. If XY is the angle bisector of DYC, prove that AB is the diameter of the circle.