susceptance slope

Dear all:

I'd like to consult a question about reactance slope parameter.

I do not understant how to define the parameter.

As we know, reactance slope parameter x=ωo/2*(dX(ω)/dω) when ω=ωo.

Based on the classical microwave passive book written by Rizzi.

It can be treated as Energy stored in the resonant circuit.

In that book, they use Q-factor to interpret this parameter.

Since the sensitvity of the resonant circuit is a function of Q,

therefore Q can be written as

Q=1/2 lim [(Δω/R)/(Δω/wω]=ωo/2R*dx/dω when ω=ωo
Δω->0

I have no idea that why we need to multiple 1/2 in this equation.

Could someone give me a hand to answer this question?

Thanks a lot.