by experiment in step by step

suppose we have three resistors connected with each other in series.

We know that in s series combination, the current through all the resistors in series is same, and the sum of potential difference across each reisitor is equal to the total potential difference across the combination.

Thus, Let the resistors be A, B and C

and them be connected across the potential difference V.

Thus using Ohm's law for each resistor and using the above two laws,

 

iA + iB + iC = V = iR

where R is the equaivalent resistance and i is the current

 

or A + B + C = R