Series and parallel combinations of resistors:
If in a circuit we have more than 1 resistors, they can be connected in different ways. The resultant resistance is also, therefore, different. Let us study these one by one.
Series combination of Resistors – When two or more resistors are joined end to end, they are said to be connected in series. You can now see this arrangement below.

In a series combination of resistors, same current passes through each resistor.
Whereas, the total potential difference across a combination of resistors in series is equal to the sum of potential difference across the individual resistors.
V=V1+V2+V3+…….
We know that V=IR
Therefore,
IR=IR1+IR2+IR3+……
R=R1+R2+R3+………
Parallel combination of resistors – If two or more resistors are connected together between any two points, then the resistors are said to be connected in parallel.

Let us now see total resistance in this case.
In parallel combination of resistors, potential difference across different resistors is same whereas current is different and total current through the circuit is given as –
I=I1+I2+I3+….
Therefore, net resistance is given as –
1/R=1/R1+1/R2+1/R3+…….