How to derive that RMS voltage equals Vm/√2 from v(t) = Vm cos(ωt+θ)

[blanik]

The question is "starting from v(t) = Vm cos(wt+theta), show analytically that the RMS value of v(t) is v(t)=Vm/sqrt(2) for any w or theta."

I'm not really sure how to begin this. Do I start by taking the integral?

 

[antonantal]

http://en.wikipedia.org/wiki/Root_mean_square