# Newbie question on equations

Discussion in 'General Electronics Chat' started by OBIE, Feb 28, 2012.

1. ### OBIE Thread Starter New Member

Feb 28, 2012
11
0
Hello,
I've been going thru the worksheets and came across some algebra manipulations that were beyond me.
The question is:
Manipulate this equation to solve for resistor value R1, given the values of R2 and Rparallel:

Rparallel = R1 R2

R1 = R2 Rparallel/ R2 − Rparallel

Appologies for the bad paste job
The question is
How do you get from the first equation to solve for R1?

2. ### MrChips Moderator

Oct 2, 2009
16,513
5,068
The answer you are looking for is

$\frac{1}{R} = \frac{1}{R1} + \frac{1}{R2}$

Hence to solve for R1

$\frac{1}{R1} = \frac{1}{R} - \frac{1}{R2}$

$\frac{1}{R1} = \frac{R2 - R}{R . R2}$

$R1 = \frac{R . R2}{R2 - R}$

3. ### OBIE Thread Starter New Member

Feb 28, 2012
11
0
Hello Mr. Chips
I am familiar with summing the reciprocals for a parallel circuit and can solve those without any problem.
The equation I am dealing with is an alternate, and it said that it is only used for 2 resistors in parallel.

Rparallel= R1xR2/R1+R2
The question is: solve for R1
R1=R2xRparallel/R2-Rparallel
The answer is plain algebraic manipulation, but this one escapes me and I'd appreciate a step by step procedure
Thank you
Obie (John)

Oct 2, 2009
16,513
5,068

5. ### crutschow Expert

Mar 14, 2008
19,384
5,399
Rp = R1R2 / (R1+R2)

Rp (R1+R2) = R1R2

RpR1 + RpR2 = R1R2

R1R2 - RpR1 = RpR2

R1(R2 - Rp) = RpR2

R1 = RpR2 / (R2 - Rp)

6. ### OBIE Thread Starter New Member

Feb 28, 2012
11
0
Hello Crutshow,
Beautiful!
Now that I see it, it's so simple. I hope with a bit more practice that I'll be able to handle most equations.
Many thanks,
Obie