一列火车以2米每秒的初速度,0,5米每秒的平方的加速度做匀加速直线运动

v=v0+at=2+0.5×3=3.5m/s
x5=v0t+1/2·a·5²=2×5+1/2×0.5×5²=16.25m
x4=v0t+1/2·a·4²=2×4+1/2×0.5×4²=12m
Δx=x5-x4=4.25m