[ \frac { R _ { 1 } } { R _ { 2 } } = \frac { R _ { 3 } } { R _ { 4 } } ]
Solve for R_1
R_{1}=\frac{R_{2}R_{3}}{R_{4}}
R_{2}\neq 0\text{ and }R_{4}\neq 0
Solve for R_2
\left\{\begin{matrix}R_{2}=\frac{R_{1}R_{4}}{R_{3}}\text{, }&R_{4}\neq 0\text{ and }R_{1}\neq 0\text{ and }R_{3}\neq 0\\R_{2}\neq 0\text{, }&R_{3}=0\text{ and }R_{1}=0\text{ and }R_{4}\neq 0\end{matrix}\right.
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R_{4}R_{1}=R_{2}R_{3}
Multiply both sides of the equation by R_{2}R_{4}, the least common multiple of R_{2},R_{4}.
\frac{R_{4}R_{1}}{R_{4}}=\frac{R_{2}R_{3}}{R_{4}}
Divide both sides by R_{4}.
R_{1}=\frac{R_{2}R_{3}}{R_{4}}
Dividing by R_{4} undoes the multiplication by R_{4}.
R_{4}R_{1}=R_{2}R_{3}
Variable R_{2} cannot be equal to 0 since division by zero is not defined. Multiply both sides of the equation by R_{2}R_{4}, the least common multiple of R_{2},R_{4}.
R_{2}R_{3}=R_{4}R_{1}
Swap sides so that all variable terms are on the left hand side.
R_{3}R_{2}=R_{1}R_{4}
The equation is in standard form.
\frac{R_{3}R_{2}}{R_{3}}=\frac{R_{1}R_{4}}{R_{3}}
Divide both sides by R_{3}.
R_{2}=\frac{R_{1}R_{4}}{R_{3}}
Dividing by R_{3} undoes the multiplication by R_{3}.
R_{2}=\frac{R_{1}R_{4}}{R_{3}}\text{, }R_{2}\neq 0
Variable R_{2} cannot be equal to 0.
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