Resonance occurs whenever the phase angle of the circuit is zero, The only way that f = 0 is if XL = XC

Note that tanϕ = \(\frac{ X }{ R}\) where X = reactance and ϕ = angle of lead or lag.

therefore, tan ϕ = \(\frac{ X_L - X_C}{ R}\) but at resonance  \(X_L = X_C\)

tan ϕ = \(\frac{0}{R}\) = 0 and ϕ = \(tan^{-1}\) = 0º