# transposing to calculate resitor values

• November 17th 2009, 04:04 AM
fjbarton
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
• November 17th 2009, 05:39 AM
Raoh
Hello(Hi)
$v=\left (\frac{R_{1}}{R_{1}+R_{2}} \right )V\Leftrightarrow v(R_{1}+R_{2})=R_{1}V\Leftrightarrow vR_{1}+vR_{2}$ $=R_{1}V\Leftrightarrow vR_{1}-R_{1}V=-vR_{2}\Leftrightarrow R_{1}(v-V)=-vR_{2}$, you know what to do next(Wink).
• November 20th 2009, 01:14 PM
fjbarton
Quote:

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

I certainly do. Ta. :)

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Frank