∵AB⊥AC
PE⊥AC,PD⊥AB,
∴PE‖AB,PD‖AC,
在△CBD中,
OM/BD=CM/CD,
同理,在△CAD中,
CM/CD=CE/CA,
则,PM/BD=CE/CA,
PM/CE=BD/CA.（更比）,
AB=AC,
PM/CE=BD/AB,
又PD‖AC,
BD/AB=BP/BC,
PM/CE=BP/BC,
在△BEC中,NP/CE=BP/BC,
∴PM/CE=NP/CE,
∴PM=PN,证毕.