Resonant Frequency, $f_p$ (Unity Power Factor)
We have derived the equation for $f_p$, which is given as
$$ f_p = f_s \sqrt{1 - { R_l^2 C \over L}} \tag{1}$$
We can rewrite the factor { R_l^2 C \over L} of Eq. (1) as
$${ R_l^2 C \over L} = { 1 \over {L \over R_l^2 C}}= { 1 \over {w L \over w R_l^2 C}}\\
= { 1 \over {w L \over R_l^2 wC}}={ 1 \over {X_L X_C \over R_l^2}}$$
and substitute ($X_L \appro X_C$):
$${ 1 \over {X_L X_C \over R_l^2}} = { 1 \over {X_L^2 \over R_l^2}} = { 1 \over Q_l^2}$$
Equation (1) then becomes
$$ f_p = f_s \sqrt{1 - { 1 \over Q_l^2}} \tag{1}$$
clearly revealing that as $Q_l$ increases, $f_p$ becomes closer and closer to $fs$.
$$ 1 - {1 \over Q_l^2} \appro 1$$
and
$$ f_p \appro f_s$$