When I execute FullSimplify on the solution of the classic linear 2nd-order ode with constant coefficients,
x''[t] + 2 \[Zeta] w0 x'[t] + wo^2 x[t] == 0,
and with the assumption that Zeta < 1, I still obtain the form with hyperbolic functions. I would like the form with cosines functions. In Mathematica 4.1, FullSimplify did that, cf http://tinyurl.com/25ff8wj.