Average Amplitude:

The average amplitude of an ac wave is the mathematical average (or mean) instantaneous current, voltage, or power computed over exactly one wave cycle or any precise whole number of wave cycles. A pure ac sine wave always has average amplitude of zero. The similar is true of a pure ac square wave or triangular wave. It is not generally the situation for sawtooth waves. You can acquire an idea of why such things are true by carefully looking at the waveforms. When you know calculus, you know that the average amplitude is the integral of the waveform computed over one full cycle.