A series circuit containing R, L and C is in resonance when the resultant reactance X = 0

i.e. the inductive reactance of the circuit is equal to the capacitive reactance.

In such a condition, impedance Z equals to the ohmic resistance R

Since the drop across the inductance leads the current by 90 degree, whereas that across the capacitor lags the current by 90 degree, the two drops are opposite to each other.

If they are made equal, then the applied voltage in the circuit is equal to the resistance drop

This condition is called Series Resonance.

I X_L=I X_C i.e. X_L=X_C i.e. 2πfL=1/2πfC at the point of Series Resonance