P是定线段AB上的点,C,D两点分别从P,B出发以每秒1cm和每秒2cm的速度沿直线AB向左运动（C在线段AP上,

设AB = L,PA = X,PB = LX
PC = 5,BD = 10
CD = PC + PB-BD = 5 + LX-10 = LX-5 = 1/2L BR />×5 = 1/2 L,PA = X = 1/2L-5,PB = 1/2L +5
点D继续运动,t秒,CD = 1 / 2L-2T,PM = CD/2-PC = 1/4L-t-7.5
PD = CD-5 = 1/2L-2t-5,PN = 1/2PD = 1/4L-t-2.5
PM-PN = -5
MN / AB =（PM + /-PN）/ AB = 1/2-（2 +10）/ L,5 / L
因此,PM -PN值不变