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Two wires of equal diameters, of resistivities ρ1 and ρ2 and lengths l1 and l2, respectively, are joined in series. The equivalent resistivity of the combination is

(1) $\frac{{\rho }_{1}{l}_{1}+{\rho }_{2}{l}_{2}}{{l}_{1}+{l}_{2}}$

(2) $\frac{{\rho }_{1}{l}_{2}+{\rho }_{2}{l}_{1}}{{l}_{1}-{l}_{2}}$

(3) $\frac{{\rho }_{1}{l}_{2}+{\rho }_{2}{l}_{1}}{{l}_{1}+{l}_{2}}$

(4) $\frac{{\rho }_{1}{l}_{1}-{\rho }_{2}{l}_{2}}{{l}_{1}-{l}_{2}}$