分别延长对角线AD,A′D′到E合E′,连接BE,B′E′不难证明
⊿ABE≌⊿A′B′E′得∠BAE＝∠A′B′D′
∵AD,A′E′分别平分∠BAC和∠B′A′C′
∴∠BAC=∠B′A′C′
∵AB=A′B′ AC＝A′C′
∴⊿ABC≌⊿A′B′C′