adding two signal
i am sorry, i am not sure that i understand your perdicament.
the signals are sinusoidal, which of course means that the coefficent is a multiple of 1 or less, correct, well that number will be dependent on the argument, which
is dependent on the freq correct, so
basically if you add two sinusoids you can have 2*A at a location or the negative or any combo in between based on where the maxima of one and the minima of the other fall
i may have not understood your question correctly
Added after 2 minutes:
sorry, i see, yes that is voltage, what you where looking at, but power
uses the formula posted by the other gentelman, sorry