往返所用时间=s/x+s/y平均速度=s/(s/x+s/y)=xy/(x+y)
时间t=s/x+s/y=s(x+y)/(xy)速度v=s/t=s/(s/x+s/y)=xy/(x+y)
t=S/x+S/yv=2s/t