去时是t1=l/(x+n)
回来时t2=l/(x-n)
来回一趟的时间t=l/(x+n)+l/(x-n)
t=l*[(x-n+x+n)/(x+n)(x-n)]
t=l*2x/(x+n)(x-n)
l=t(x+n)(x-n)/(2x)