## Elementary and Intermediate Algebra: Concepts & Applications (6th Edition)

$r=\displaystyle \frac{2V-IR}{2I}$
The goal is to isolate the wanted variable on one side of the equation. First of all, the denominator must not be 0, $r\displaystyle \neq-\frac{R}{2}.$ Multiply both sides with $(R+2r)$ $I(R+2r)=2V$ $IR+2Ir=2V\qquad$ ... subtract $IR$ $2Ir=2V-IR\qquad$ ... divide with $2I$ $r=\displaystyle \frac{2V-IR}{2I}$