25.5.4 Power Reduction and Half-Angle Uses
Core ideaRun the cos2α formula backward and you can bring the squares in sin² and cos² 'down a level', which makes integration and simplification easier.
Module goal. We build the rules for combining known angles into new ones — starting from the sum and difference formulas to reach double-angle and auxiliary-angle forms — so messy trig expressions can be simplified, combined, and evaluated.