[tex]E_s=\sum\limits^n_{i=1} {E_i} \ =nE\\r_s=\sum\limits^n_{i=1} {r_i} \ =nr\\I=\frac{E_s}{R+r_s}=\frac{nE}{R+nr}=\frac{nE}{n(\frac{R}{n}+r) }=\frac{E}{\frac{R}{n}+r} \\I=\frac{E}{\frac{R}{n}+r}[/tex]
