x=0.5at²得到a=x/(0.5t²)=0,5m/[0.5*（1s)²]=1m/s²
v3=at3=1m/s²*3s=3m/s
v5=at5=1m/s²*5s=5m/s
v6=at6=1m/s²*6s=6m/s
第六秒内的平均速度v=(1/2)(v5+v6)=5,5m/s