\(\therefore \,\,\,{\left( {\frac{E}{{{R_1}\, + \,r}}} \right)^2}\,{R_1}\,\, = \,\,{\left( {\frac{E}{{{R_2}\, + \,r}}} \right)^2}\,{R_2}\)
\( \Rightarrow \,{R_1}\,(R_2^2\, + \,{r^2}\, + \,2{R_2}^r)\,\, = \,\,{R_2}\,(R_1^2\, + \,{r^2}\, + \,2{R_1}r)\)
\( \Rightarrow \,R_2^2{R_1}\, + \,{R_1}{r^2}\, + \,2{R_1}\,{R_2}r\,\, = \,\,R_1^2\,{R_2}\, + \,{R_2}{r^2}\, + \,2{R_1}\,{R_2}r\)
\( \Rightarrow \,{({R_1}\, - \,{R_2})^2}\, = \,\,({R_1}\, - \,{R_2})\,{R_1}\,{R_2}\,\,\, \Rightarrow \,\,r\,\, = \,\,\sqrt {{R_1}\,{R_2}} \)