Let R,C,L be the re-stance capacitance and inductance connected parallel to the source of voltage V. Admittare $$gamma=gamma_R+gamma_c+gamma_2$$ $$=dfrac{1}{R}+jcw+dfrac{1}{jLw}$$ Resonance occurs when the current and voltage are in phase. To achieve this ,the imaginary past of admittance (or importance) must be zero $$ herefore jcw+dfrac{1}{jLw}=0,w=dfrac{1}{sqrt{Lc}}=2pi f_R$$. Resonant frequency the frequency at which the voltage and current are in phase in the circuit We know $$w=dfrac{1}{sqrt{Lc}}=2pi f_{R1}$$ $$ herefore =dfrac{1}{2 pi sqrt{Lc}}$$