How to reverse formulas?

Thread Starter

SimplyElectronics

Joined Jul 14, 2016
16
Hi all,

I am struggling to change the subject of the formula below:

VOUT = 1.25 * ( 1 + R2/R1 )

I know VOUT = 5

and I know the value of R1


I need to rearrange this equation so that R1 is the subject which will then allow me to work out the value of R2


Could anyone help explain how I change the formula?


Thank You
 

Jony130

Joined Feb 17, 2009
5,487
Members are not supposed to just work a student's problem for them -- significant learning is seldom achieved.
VOUT = 1.25 * ( 1 + R2/R1 )

1.25 * ( 1 + R2/R1 ) = VOUT

( 1 + R2/R1 ) = VOUT/1.25

R2/R1 = VOUT/1.25 - 1

R2 = (VOUT/1.25 - 1) * R1


Or we know that the voltage across R1 is 1.25V so IR1 = (1.25V/R1) and we also knows that

Vout = VR1+VR2 = 1.25V + VR2

VR2 = 5V -1.25V = 3.75V so R2 = VR2/IR1 = 3.75V/(1.25V/R1)
 

Thread Starter

SimplyElectronics

Joined Jul 14, 2016
16
VOUT = 1.25 * ( 1 + R2/R1 )

1.25 * ( 1 + R2/R1 ) = VOUT

( 1 + R2/R1 ) = VOUT/1.25

R2/R1 = VOUT/1.25 - 1

R2 = (VOUT/1.25 - 1) * R1
Thank You!

Could you perhaps explain or point me in the right direction to where I could understand the process for which the equations were changed?
 

WBahn

Joined Mar 31, 2012
29,979
Thank You!
Yes, it's always nice when someone does your homework for you so that you can copy their work and turn it in as your own, isn't it?

Could you perhaps explain or point me in the right direction to where I could understand the process for which the equations were changed?
It's called "algebra" and is commonly the focus of a couple of years of math class in the 7th to 9th grade range. Most colleges also teach it under titles such as "Algebra for College Students" or "College Algebra". There are also lots of online courses (both free and paid) that cover algebra.
 

wayneh

Joined Sep 9, 2010
17,496
Could you perhaps explain or point me in the right direction to where I could understand the process for which the equations were changed?
If you are past grade school and asking this question, I would advise you to take an algebra class to catch up as soon as possible. I don't mean to sound condescending but this is a very easy and very basic type of problem. You won't get far in electronics, or any scientific field, without being able to perform this sort of task.

If you're still in grade school, then you now have something to look forward to. ;)
 
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