1.v1=a1*t1=5m/s,v1=a2*t2,得a2=2.5m/s2
2.s=1/2v1*t1+1/2v1*t2+v1*(15-5-2)=57.5m
3.由0s时0m/s到5s时5m/s,由5s到13s不变,由13s时5m/s到15时0m/s