Let ABC be an acute-angled triangle with AC=BC, and let O be the circumcentre and F the foot of the altitude through C. Furthermore, let X and Y be the feet of the perpendiculars dropped from A and B respectively to (the extension of) CO. The line FO intersects the circumcircle of FXY a second time at P. Prove that OP<OF. inequalitiesgeometrycircumcirclegeometry unsolved