# Thread: transposing to calculate resitor values

1. ## transposing to calculate resitor values

in a circuit for a potential divider which calculates voltage drops, i nned to transpose it so that i can calculate resistances.

The existing formula is

v=(R1/RT)(V)

where v is voltage drop across a resistor (9.38 Volts) and V is source voltage(10 volts), R1=75k ohm, R2=5k ohm, RT=R1+R2. How would I make R1 the subject? Thanks

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Frank

2. Hello
$\displaystyle v=\left (\frac{R_{1}}{R_{1}+R_{2}} \right )V\Leftrightarrow v(R_{1}+R_{2})=R_{1}V\Leftrightarrow vR_{1}+vR_{2}$$\displaystyle =R_{1}V\Leftrightarrow vR_{1}-R_{1}V=-vR_{2}\Leftrightarrow R_{1}(v-V)=-vR_{2}, you know what to do next. 3. Originally Posted by Raoh Hello \displaystyle v=\left (\frac{R_{1}}{R_{1}+R_{2}} \right )V\Leftrightarrow v(R_{1}+R_{2})=R_{1}V\Leftrightarrow vR_{1}+vR_{2}$$\displaystyle =R_{1}V\Leftrightarrow vR_{1}-R_{1}V=-vR_{2}\Leftrightarrow R_{1}(v-V)=-vR_{2}$, you know what to do next.
I certainly do. Ta.

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Frank