证:作PQ‖AC,交BD于Q.
∵PF⊥AC,BD⊥AC.∴四边形PFDQ为矩形.PF=QD
∵∠ABP=∠ACB=∠QPB,∠BEP=∠PQB=90°,BP共用.
∴⊿EBP≌⊿BPQ.PE=BQ
∴BD=BQ+QD=PE+PF