Resistors in Parallel 

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Components connected in Parallel are wired together as shown in the diagram.
For three resistors ...
R_{total} = 1 / ( 1 / R_{1} + 1 / R_{2} + 1 / R_{3} )
For just two resistors ...
R_{total} = 1 / ( 1 / R_{1} + 1 / R_{2} )
You need to be good at fractions or skilled with your calculator!
On a calculator, to get R_{total} press these keys ... 1 / ( 1 / R_{1} + 1 / R_{2} ) =
Problems like this need to be solved in stages by breaking the problem into simpler parts.
In the diagram below, add R1 + R2 to work out the series resistors.
Then use the parallel resistors formula to work out R3 in parallel with the result of (R1 + R2).
In the next example, work out each of the parallel resistors first and then add the two answers together.
(R1 and R2) in parallel added to (R3 and R4) in parallel.
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