Can I take the RMS of DC and AC, or only AC? And why isn't it exactly equal to Peak/sqrt(2)? My teacher said there is a pretty big margin for error when calculating RMS, but he didn't state why.
If the signal were half-rectified, then the square of the signal appears as a constant value: V^2. So the average of this would still be V^2, and upon taking the square root we would get |V| again.