Why L-C circuit is considered as tank circuit?

The amplitude of current in series LCR circuit connected to an AC frequency "omega" is given by i_m = v_m/(sqrt(R^2+(X_L-X_C)^2) Where X_L and X_C are inductive and capacitative reactances respectively and "v_m" is amplitude of voltage. Starting fron this equation show that sharpness of resonance in the circuit is equal to the quality factor of the circuit.