已知V1=25m/s,V2=50m/s,求:V平均=?
解 :设全程的距离为S,
V平均=S/(t1+t2) =S/(S/3V1+2S/3V2)=3V1V2/(V2+2V1)=3*25*50/(50+2*25)=37.5m/s