# Finding unknown resistor in parallel

#### newtoelectronics

Joined Jan 11, 2009
2
I'm sure this is a very simple problem for everyone here, but I'm unsure on how to solve this particular problem in my text.

The problem is asking me if RT is 400 Ohms, R1 and R2 are 2k ohms each, what is R3 if these resistors are connected in parallel?

I know to calculate RT if the resistors are known using the reciprocal of 1/r for more than 2 different value resistors, r1*r2/r1+r2 for two resistors in //, if I have say 3 equal resistors of 3 ohms each for example, I can divide that by the # of resistors in // and get 1 ohm as Req.

So how can I solve for an unknown resistor in //? What I have found to be a problem is when I am asked to solve for something that wasn't covered in the text. I am unsure what formula to use or do not know how to derive a formula. Tips and help is greatly appreciated in advance!

#### t_n_k

Joined Mar 6, 2009
5,455
Use this

1/RT=1/R1 + 1/R2 +1/R3

Re-arranging

1/R3 = 1/Rt - 1/R1 - 1/R2

and so on.

For N resistors in parallel

1/RT= 1/R1 + 1/R2 + .....+1/RN