过P作PM∥AC交AB于M,
过P作PN∥AB交AC于N,
有AM=PN,AN=PM.
△PBM中,PM+BM>PB(1)
△PCN中,PN+CN>PC(2)
(1)+(2)得:
PM+BM+PN+CN>PB+PC,
(PM+CN)+(PN+BM)>PB+PC
∴AC+AB>PM+PN.
过P作PM∥AC交AB于M,
过P作PN∥AB交AC于N,
有AM=PN,AN=PM.
△PBM中,PM+BM>PB(1)
△PCN中,PN+CN>PC(2)
(1)+(2)得:
PM+BM+PN+CN>PB+PC,
(PM+CN)+(PN+BM)>PB+PC
∴AC+AB>PM+PN.