$n$ equal cell having emf $E$ and internal resistance $r$, are connected in a circuit of a resistance $R$ . Same current flows in circuit either they are connected in series or parallel, if
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$\frac{\mathrm{nE}}{\mathrm{R}+\mathrm{nr}}=\frac{\mathrm{E}}{\frac{\mathrm{r}}{\mathrm{n}}+\mathrm{R}}$
$\Rightarrow \mathrm{r}+\mathrm{Rn}=\mathrm{R}+\mathrm{nr}$
$\Rightarrow \mathrm{r}=\mathrm{R}$
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