# Solving for an indicated variable where all the variables are under 1 in a fraction [duplicate]

Possible Duplicate:
Solving a literal equation containing fractions.

1/R=(1/R1)+(1/R2) solve for R1. I can't figure out what to do, I always end up where I have to either get rid of R1 or an answer that doesn't work when plugging it back in.

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## marked as duplicate by MJD, Chris Eagle, t.b., sdcvvc, Ｊ. Ｍ.Aug 29 '12 at 10:35

Hint:

1) isolate 1/R1

2) simplify both sides of that isolation

3) figure out the two conditions for this to work

If this is not clear, ask again.

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$$\frac{1}{R}= \frac{1}{R_1} +\frac{1}{R_2}$$

$$\frac{1}{R_1}=\frac{1}{R}-\frac{1}{R_2}$$

$$R_1=\frac{1}{\frac{1}{R}-\frac{1}{R_2}} = \frac{1}{\frac{R_2-R}{R\, R_2}}= \frac{R\, R_2}{R_2-R} .$$

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Thank you for showing me how to get to the answer because the book I'm using did not give me the answer to the question so I was not sure if any of the answers I got were right. –  Dragoon2375 Aug 27 '12 at 4:10