- #1

seanmcgowan

- 35

- 0

## Homework Statement

What is the value of the unknown resistor in the circuit shown if the equivalent resistance for the circuit is 21.50 ohms.

## Homework Equations

Series= R1+R2+R3...

Parllel= (1/R1)+(1/R2)+(1/R3)...

## The Attempt at a Solution

First off, I'm sorry for the horrendous drawing I did, but its better than nothing i guess.

I have done several of these problems and I am STILL stuck on every single one of them.

What I did was I drew several different possibilities of series and parallel circuits, but none of them came anywhere near the 21.50 ohms.

First I drew a pic, that went like this: (series) 7.5 and 9.1 then parallel 6.81 and ? then series again 9.1 and 3.6. However, just adding the series circuits together I came up with 29.3, and that was before the Parallel circuit.

Then I tried leaving the very first circuits (the first 9.1 and 7.5) and series, and making everything else parallel:

1/9.1= .11

1/3.6= .29

1/6.81= .15

.11+.29+.15= .55

1/.55= 1.82

7.5+9.1= 16.6

16.6+1.82= 18.42

Im fairly certain that since the unknown factor would be prallel you can't just subtract 18.42 from 21.50

How do I find this? And, am I even close to guessing the correct parallel and series combination? I'm clueless, an help is appreciated.