v0=12m/s
a=3m/s^2
3秒内通过的位移
x = v0t-1/2at^2 = 12*3-1/2*3*3^2 = 36-27/2 = 45/2 m
制动时间t = v0/a = 12/3 = 4s
刹车后8s内通过的位移 = 制动总距离 = v0^2/(2a) = 12^2/(2*3) = 24m