In such complex circuits, each relationship for calculating resistors in series and in parallel must be used separately. In the above example, the total series resistance of \({R_1}\) and \({R_2}\) ...

In other words, the voltages around the circuit add up to the voltage of the supply. The total resistance of a number of resistors in series is equal to the sum of all the individual resistances.

In actual use, though, almost all circuits have some reactance; and impedance is almost always the better term. Like impedance, resistance is also measured in ohms, and represented by the letter “R.” ...