I have a trivial question about derivation of characteristic quantity of Crystal.

See http://www.yoketant.com.tw/yoketant/terms.htm

It is well known that Loaded Resonance Resistance(=RL) of Crystal can be approximated as RL≈R1*(1+C0/CL)².

R1 ; Motional Resistance(=Equivalent Series Resistance)
C0 ; Shunt Capacitance
CL ; Load Capacitance

Here my question is how to derive this approximated formula for RL.