连接OD,DB,CB,过D作DG⊥AB于GC,D在⊙O上,AB是直径,DE是⊙O的切线,DE⊥AE∴AC⊥BC AD⊥BD DE⊥OD∴AE‖OD BC‖DE △ADB≌△AGD △AGD=△AED △AEF ≌△DOF AF:DF=AE:OD AG=AE AC:OG=AB:OD=2:1∵AC:AB=3:5∴OG:AO=3:5 AG:AO=8:5 AO=OD AG:OD=8:5 AF:DF=8:5