证明：连接DE∵四边形ABED内接于圆O∴∠CDE＝∠ABC∵AB＝AC∴∠C＝∠ABC∴∠C＝∠CDE∴EC＝DE∵BD平分∠ABC∴∠ABD＝∠CBD∴弧AD＝弧ED∴AD＝DE （等弧对等弦）∴AD＝EC
