∵BD=AD CD=ED ∠ADC=∠ADB
∴△ADC≌△BDE
∴∠C=∠BED
∵∠BED=∠AEF
∴∠C=∠AEF
∵∠C+∠CAD=90°
∴∠AEF+∠CAD=90°
∴∠AFB=90°
∴BF⊥AC