解答
$\frac{1}{R_{\text{eq}}} = \frac{1}{R_1} + \frac{1}{R_2} \implies R_{\text{eq}} = \frac{R_1 R_2}{R_1 + R_2}$.
解説
In parallel connections, voltage across both branches is identical, and total current is the sum of branch currents ($I = I_1 + I_2$).