Series combination of resistors

“Series combination of resistors”:
when two or more resistors are combined together end to end. This combination is called series combination of resistors.

In a figure shows the series combination of three resistors of resistance R1, R2 and R3 respectively. Equal current I ampere is flowing through each resistor but potential difference produce across different registers are different as V1, V2 and V3.

we know that in series combination: Applied potential = sum of all P.D across 
                                   different registers

V = V1 + V2 + V3 --------(1)

By ohm's law:
P.D across first resistor, V1 = R1×I

P.D across second resistor, V2 = R2×I

P.D across third resistor, V3 = R3×I

If the equivalent resistance of the combination be “R” ohm. Thus we can replace all the resistance by single resistor of resistance R.
then,
potential difference across equivalent register, V = R×I

All these value put in eqn1

R×I = R1×I + R2×I + R3×I

R×I = I ( R1+ R2+ R3)

[ R =  R1+ R2+ R3 ]