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ASEmech

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I'm currently studying Ohm's Law. Within Ohm's Law, I'm studying Resistance in a parallel circuit. The formula I'm given is:

R0=1/(1/R1) + (1/R2) = (R1 x R2)/(R1 + R2)

...where R0 (read R-sub0) is a combination of resistances R-sub1 and R-sub2.

I thought the sum of the resistances in any circuit was R0=R1+R2. Why is 1 divided by (1/R1) + (1/R2) and in the "denominator portion" of the formula, why is 1 divided by R1 (same question for 1 divided by R2) and then added to 1/R2? Also, how do you get from R0=1/(1/R1) + (1/R2) to (R1 x R2)/(R1 + R2)?