2006 Romanian NMO Problems/Grade 7/Problem 1
Let be a triangle and the points and on the sides respectively , such that . Let be a point on the line . Prove that the lines and are perpendicular if and only if is the interior angle bisector of . Let L be a point on BC such that N is the midpoint of LC, then 2CN=LC, the given information is the same as LC/BC=AM/AB, applicating Thales theorem it follows that ML is parallel to AC.
Let R be the point on MN such that MN=NR, in view of MN=NR and LN=NC it follows that RLMC is a parallelogram, implying that CR is parallel to ML, but we know that ML is parallel to AC, then A,C,R are collineal.
MN is perpendicular to PN if and only if NP is the perpendicular bisector of MC if and only if PN is the angle bisector of MPR if and only if PN is the angle bisector of MPC, as requiered.