Đáp án:
Ta có vì R1 // R2 nên:
$\begin{array}{l}
\dfrac{1}{{{R_{12}}}} = \dfrac{1}{{{R_1}}} + \dfrac{1}{{{R_2}}}\\
\Leftrightarrow \dfrac{1}{{{R_{12}}}} = \dfrac{{{R_2}}}{{{R_1}.{R_2}}} + \dfrac{{{R_1}}}{{{R_1}.{R_2}}} = \dfrac{{{R_1} + {R_2}}}{{{R_1}.{R_2}}}\\
\Leftrightarrow {R_{12}} = \dfrac{{{R_1}.{R_2}}}{{{R_1} + {R_2}}}\left( {dpcm} \right)
\end{array}$
