Resistance in Series with 2 unknown resistors

Thread Starter


Joined Sep 26, 2020
Im stuck on a question and I need help finding the right formula for series a ckt.

Find R1, R2 and It
R1= unknown
R2= unknown
R3= 50 ohms
It(current total)= unknown

Voltage total = 9v
VR1 (v-drop)= 4v

Which formula do I use:
V= I x R
R= V/I
or I-Division?
Last edited:


Joined Oct 2, 2009
Use Ohm’s Law,

I = V / R

V = I x R

R = V / I

All three are variations of Ohm’s Law.
The first one is Ohm’s Law. The other two are corollaries of Ohm’s Law.


Joined Mar 2, 2015
Instead of worrying about "finding the right formula" learn to proceed logically, step by step, from what you do know (the resistance of R3, and the voltage across it) to what you don't know:

Given the value of R3 and knowing the voltage across it, can you calculate the current through it?
Knowing the current through R3, how much current is flowing through the other two resistors, given they're all in series?
Knowing the current through the remaining two resistors and the voltages across them, can you calculate their values?

Do that, and you'll have your answers.

This kind of "logical progression" mindset will be called on again and again in design work. Learn to master it.


Joined Mar 31, 2012
Each resistor has three parameters that describe it (voltage across it, current through it, and resistance of it). Can you find the value of all three parameters for any of the resistors given the information you have?


Joined Oct 2, 2009
Since you do not seem to be able to work through the problem on your own, here is a systematic approach.

You have three resistors. Each resistor has a current and voltage, i.e. there are three parameters R, V and I
Write out Ohm's Law for each resistor.

For example, for R1,

I1 = V1 / R1

Do this for all three resistors, R1, R2, R3. Hence there are nine values in total that you need to quantify.
Create a list of all nine quantities.
Write down all the information you know and can deduce for each of the nine quantities.
Where there are more than one unknown values, write down the pertinent Ohm's Law formula.


Joined Mar 31, 2012
so would it be... 2volts divided by50 ohms to get the current going through R3? which is the total current
Correct. So now you have one more piece of information than you did. So, with that in mind, go back and answer the same question I asked before, namely can you find the value of all three parameters for any of the resistors given the information you have?