x1= 1/2a1(t+2)^2
x2 = 1/2a2t^2
x1=x2
1/2a1(t+2)^2 = 1/2a2t^2
a1(t+2)^2 = a2t^2
2(t+2)^2 = 4.5t^2
{(t+2)/t}^2 = 2.25
(t+2)/t = 1.5
0.5t=2
t=4
B車開出4s追上A車
△x = x1-x2
= 1/2a1(t+2)^2 - 1/2a2t^2
= 1/2*2(t+2)^2 - 1/2*4.5t^2
= (t+2)^2 - 2.25t^2
= -1.25t^2+4t+4
= -1.25(t^2-3.2)+4
= -1.25(t-1.6)^2 + 3.2+4
= -1.25(t-1.6)^2 + 7.2
≤7.2
B車追上A車之前,兩車相距的最大距離是7.2m
v1=a1(t+2)=2t+4,其中t≥-2
v2=a2t=4.5t,其中t≥0
v1-t圖像：為一端點在(-2,0）,向右上方傾斜,在縱坐標正半軸上截距4的射線；
v2-t圖像：為一端點在原點(0,0）,向右上方傾斜,過(1,4.5）的射線.