How to solve this differential equation? (harmonic oscillator)

*To*: mathgroup at smc.vnet.net*Subject*: [mg113041] How to solve this differential equation? (harmonic oscillator)*From*: MathProg <denis1503 at gmail.com>*Date*: Tue, 12 Oct 2010 04:24:13 -0400 (EDT)

I have the differential equation: x'' + w^2*x(t) = 0. I don't know how to solve it in Mathematica. I know that the answer is: A * cos( w*t + phi ) .but how I can get it in Mathematica? Can anyone help me with that?