Let EN be perpendicular to AB, because DE is the perpendicular bisector of BC, connecting CE, so CE=BE, and because AE is the bisector of angle BAC, EM is perpendicular to AC, and EN is perpendicular to AB, so EN=EM, and angle ENB= angle EMC = 90, so triangle BEN is congruent with triangle CEM, so BN=CM, and because An = AM, AB =