Question

N/(v+u)=N1/v,N/(v-u)=N2/v 解得N=2N1N2/(N1+N2) ,是如何解的

Answer 1

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)