⊿BDE≌⊿CEF
证明:∠CEF=180°-∠DEF-∠BED;∠BDE=180°-∠B-∠BED.
又∠B=∠DEF.则∠CEF=∠BDE.
又∠B=∠C;BD=CE.所以⊿BDE≌⊿CEF(ASA).