证明:延长BE使HE=EF,连接AFBE⊥AC,HE=EF,易知△AHF是等腰三角形∠HAE=∠EAF,AH=AFAD⊥BC,BE⊥AC∠HDB=∠AEH=90°因∠HBD+∠BHD=∠HAE+∠AHE=90°,∠BHD=∠HAE所以∠HBD=∠AHE又∠HAE=∠EAF∠HBD=∠EAF,BE=AE,∠BEC=∠AEF=90°RT△BEC≌RT△AEF(ASA) BC=AFAH=AFBC=AH,BC=2BDAH=2BD