先假设两条中线AD,BE交与P点 连接CP,取AB中点F连接PF PA+PC=2PE=BP PB+PC=2PD=AP PA+PB=2PF 三式相加 2PA+2PB+2PC=BP+AP+2PF 3PA+3PB+2PC=2PF 6PF+2PC=2PF PC=-2PF 所以PC,PF共线，PF就是中线 所以ABC的三条中线交于一点P
