LETABCDBE A QUADRILATERALABCDAND A POINTP IN THE PLANE OF THE QUADR...

2. LetABCDbe a quadrilateralABCDand a pointP in the plane of the quadrilateral. LetM, N be onthe sidesAC, BDrespectively such thatP M k BC, P N k AD.AC meetsBD atE. Prove that theorthocenter of trianglesEBC, EAD, EM N are concurrent if and only ifPis on the lineAB.Đỗ Thanh Sơn