\frac{1}{R_{(1)}} = \frac{1}{R_{1}} + \frac{1}{R_{2}} = \frac{9 S}{2 ρ l} ⟹ R_{(1)} = \frac{2 ρ l}{9 S}

${\frac{1}{R_{(1)}}=\frac{1}{R_1}+\frac{1}{R_2}=\frac{9S}{2\rho l} \Longrightarrow R_{(1)} =\frac{2\rho l}{9S}}$

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