证明：∵AD平分∠BAC∴∠BAD＝∠CAD∵DE∥AC∴∠ADE＝∠CAD∴∠ADE＝∠BAD∴AE＝DE∵DF∥AB∴平行四边形AEDF（两组对边平行）∴AF＝DE，DF＝AE∴AE＝DE＝AF＝DF∴菱形AEDF∴AD⊥EF
