延长AM到点E,使得:ME = MD,连接CE.
则CM是DE的垂直平分线,可得:MD = ME ,CD = CE ;
而且,AB = AD ,可得:∠E = ∠CDE = ∠ADB = ∠B .
因为,∠ACE = 180°-∠CAE-∠E = 180°-∠BAD-∠ADB = ∠B = ∠E ,
所以,AC = AE .
AM = (1/2)(AM+AM) = (1/2)(AM-MD+ME+AM) = (1/2)(AD+AE) = (1/2)(AB+AC) .
延长AM到点E,使得:ME = MD,连接CE.
则CM是DE的垂直平分线,可得:MD = ME ,CD = CE ;
而且,AB = AD ,可得:∠E = ∠CDE = ∠ADB = ∠B .
因为,∠ACE = 180°-∠CAE-∠E = 180°-∠BAD-∠ADB = ∠B = ∠E ,
所以,AC = AE .
AM = (1/2)(AM+AM) = (1/2)(AM-MD+ME+AM) = (1/2)(AD+AE) = (1/2)(AB+AC) .