N/(v+u)=N1/v
所以：Nv=N1*v+N1*u
(N-N1)v=N1*u (1)
N/(v-u)=N2/v
所以：Nv=N2*v-N2*u
(N-N2)v=-N2*u (2)
(1)/(2)得：(N-N1)/(N-N2)=N1/(-N2)
化简上式得：N=2N1N2/(N1+N2)