Resonance occurs whenever the phase angle of the circuit is zero, The only way that f = 0 is if XL = XC
Note that tanϕ = XR\frac{X}{R}RX where X = reactance and ϕ = angle of lead or lag.
therefore, tan ϕ = XL−XCR\frac{X_L - X_C}{R}RXL−XC but at resonance XL=XCX_L = X_CXL=XC
tan ϕ = 0R\frac{0}{R}R0 = 0 and ϕ = tan−1\tan^{-1}tan−1 = 0º
