汽车在全路程的平均速度=总路程÷总时间
=﹙S1＋S2﹚÷[﹙S1／V1﹚＋﹙S2／V2﹚]
=2S1÷[S1﹙V1＋V2﹚／﹙V1·V2﹚]
=2V1·V2／﹙V1＋V2﹚