Let ABC be a triangle inscribed in a circle (O). d is the tangent at A of (O),P is an arbitrary point in the plane. D,E,F are the projections of P on BC,CA,AB. Let DE,DF intersect the line d at M,N respectively. The circumcircle of triangle DEF meets CA,AB at K,L distinct from E,F. Prove that KN meets LM at a point on the circumcircle of triangle DEF.Trần Quang Hùng concurrentgeometrycircumcircle