a(t)=0.8+1.6t
v(t)=2.5+0.5t(0.8+0.8+1.6t)=0.8*t^2+0.8t+2.5
s=(0,t)∫v(t)dt=4/15*t^3+0.4t^2+2.5t=3.8
解得t≈1.148