Thank you, it is what I need but with the number "2" on to the root too, this is what I have done:

((sol = s /. Solve[s^2 + (R/L) s + 1/(L*C1) == 0, s] // Expand) /.Sqrt[x__]/(a_ *Sqrt[y_]) :> Sqrt[x/(y *a^2)])/.Sqrt[(x_+y1_)/(y_ *a_^2)]:>Sqrt[x/(y *a^2)+y1/(y *a^2)]

it give me this:

I'm so close to have the expression I want, pls how can I put the "(1/2)" on to the root y try to put this "#" but the command doesn't recognize the number.

Thank you for helping me.

For printing (only) purposes you may write

bb /. four -> "4"

Thank you guys, you're the boss we got it the expression, well something like that, this is the final code:

aa = ((sol = s /. Solve[s^2 + (R/L) s + 1/(L*C1) == 0, s] // Expand) /. 
    Sqrt[x__]/(a_*Sqrt[y_]) :> Sqrt[x/(y*a^2)]) /. 
  Sqrt[(x_ + y1_)/(y_*a_^2)] :> Sqrt[x/(y*a^2) + y1/(y*a^2)];
bb = aa /. Sqrt[x__] -> xx /. xx -> 2 yy /. 
  yy -> Sqrt[(-(4/(four*C1*L)) + R^2/(four*L^2))];
  bb /. four -> "4" 

and this is the final result:

Finally I just put "four" as divisor, I'm new in this and you help me, thousand thanks to you guys it is what I want to do.

Note: where can I found more information about pure functions in wolfram or symbolic manipulation? the examples of Wolfram introduction are basics.