连接AP
∵PE⊥AB
∴S△ABP＝AB×PE/2
∵PF⊥AC
∴S△ACP＝AC×PF/2
∵BD⊥AC
∴S△ABC＝AC×BD/2
∵S△ABC＝S△ABP+ S△ACP
∴AC×BD/2＝AB×PE/2+ AC×PF/2
∵AB＝AC
∴BD＝PE+PF