Answer
$R_1=\frac{1}{\frac{1}{R_2}-\frac{1}{R}}$
Work Step by Step
$\frac{1}{R}=\frac{1}{R_1}+\frac{1}{R_2}$
$\frac{1}{R_1}=\frac{1}{R_2}-\frac{1}{R}$
$1=R_1(\frac{1}{R_2}-\frac{1}{R})$
$R_1=\frac{1}{\frac{1}{R_2}-\frac{1}{R}}$
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