设EF交AD于O
∠BAD=∠DAC,∠AOE＝∠AOF,AO=AO ==》⊿AOE≌⊿AOF
==》∠AEO＝∠AFO＝∠CFM
∠AEO＝∠B＋∠M,∠ACB＝∠CFM＋∠M
==》∠ACB＝∠B＋∠M＋∠M
==》∠M＝1/2（∠ACB－∠B）