设∠ABC＝90º
设BD＝1,则AB＝BC＝2 AD＝√5 BF＝1×2/√5 BF/AD＝2/5＝0.4
[如果∠C＝90º,BF的计算麻烦一些,对⊿BDF用余弦定理即可.]能否再详细些抱歉，我把题目当成BF⊥AD做了。只好重做啦！如图，设AC＝2.则AD＝√5,CF＝2/√5AF＝4/√5 DF＝√5-4/√5＝1/√5对⊿BDF用余弦定理.BF＝2√2/√5BF/AD＝2√2/5≈0.5657答案有误，∠ABC＝90°，与图不符