解方程组N/(v+u)=N1/v ,N/(v-u)=N2/v 求出N
问题描述：
解方程组N/(v+u)=N1/v ,N/(v-u)=N2/v 求出N
求出N
答案是N=2N1N2/(N1+N2)
答
由1式：N=N1(1+u/v),即u/v=N/N1-1
由2式：N=N2(1-u/v),即u/v=1-N/N2
两式相减,消去u/v:
N/N1-1-1+N/N2=0
N(1/N1+1/N2)=2
N=2N1N2/(N1+N2)