ABSTRACT
This chapter helps the readers to derive double angle formulae and to change products of sines and cosines into sums or differences and sums or differences of sines and cosines into products. Compound angle (or sum and difference) formulae and double angles are further commonly used identities. Compound angles are required for example in the analysis of acoustics (where a beat is an interference between two sounds of slightly different frequencies), and with phase detectors (which is a frequency mixer, analogue multiplier or logic circuit that generates a voltage signal which represents the difference in phase between two signal inputs). Two periodic functions of the same frequency may be combined by plotting the functions graphically and combining ordinates at intervals, or by resolution of phasors by drawing or calculation.
