Resistors are combined to increase or decrease total resistance in the circuit. Resistors can be combined in two ways; series and parallel.
Resistors in Series
In this type of combination, resistors are connected end to end consecutively. The combination of three resistors of resistances

Consider a battery of voltage
From Ohm’s law,
From
Resistors in Parallel
In this type of combination, resistors are connected between the same two points. The combination of three resistors of resistances

Consider a battery of voltage
Let
From Ohm’s law,
Let
For
Thus, in parallel combination, the reciprocal of the equivalent resistance is equal to the sum of the reciprocal of individual resistances. And, the equivalent resistance is always less than the individual resistance. So, to decrease the resistance in the circuit, resistors are combined in series.