Question: Resonance peak: formula for frequency

I want to get the formula for frequency at resonance peak (   ωn=ω*sqrt(1 - 2*ζ2)   ) in Maple 15, starting from the transfer function

                      G(s) := 1/(1+2*zeta*s/omega[n]+(1/(omega[n])^2)*s^2);

I wrote these lines but I haven't obtained the results I wanted:

Abs := simplify(normal(rhs(%)));

I need help. The resonance peak should exist for zeta < 1/sqrt(2) if I'm not wrong.. 

Thanks everybody!  


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