Observe the circuit:

In a series RLC circuit contains a resistor, an inductor and a capacitor.

The source voltage is the phasor sum made up of three components .

If , then the overall circuit reactance is inductive giving lagging phase angle.

The instantaneous voltage across a pure resistor, is in-phase with current.

The instantaneous voltage across a pure inductor,   leads the current by .

The instantaneous voltage across a pure resistor, lags the current by .

Phasor diagram for the individual vectors:

Resultant of three diagrams:

Therefore,

The input sine voltage is .

Compare the function with .

Here and .

and are in parallel.

can be written in polar form is .