\(\mathrm{V}_{1}=\mathrm{i} \mathrm{R}_{1}=\mathrm{x} l_{1}\)
When key is at \(( 3)\)
\({{\rm{v}}_2} = {\rm{i}}\left( {{{\rm{R}}_1} + {{\rm{R}}_2}} \right) = {\rm{x}}{l_2}\)
\(\frac{\mathrm{R}_{1}}{\mathrm{R}_{1}+\mathrm{R}_{2}}=\frac{l_{1}}{l_{2}} \Rightarrow \frac{\mathrm{R}_{1}}{\mathrm{R}_{2}}=\frac{l_{1}}{l_{2}-l_{1}}\)