\frac{1}{R_{(2)}} = \frac{1}{R_{1}} + \frac{1}{R_{2}} = \frac{2}{R_{1}} = \frac{4 S}{ρ l} ⟹ R_{(2)} = \frac{ρ l}{4 S}

${\frac{1}{R_{(2)}} = \frac{1}{R_1}+\frac{1}{R_2}=\frac{2}{R_1}=\frac{4S}{\rho l} \Longrightarrow R_{(2)} = \frac{\rho l}{4S}}$