25.3.4 Memory Tricks and Simplifying with Reduction Formulas
Core idea'Odd changes the name, even keeps it; the sign comes from the quadrant' lets you boil any angle down to a familiar acute one before you evaluate.
Module goal. We upgrade trig functions from the right triangle to the coordinates of a point on the unit circle, so they make sense for any angle, and we use the circle's symmetry to read off the reduction formulas all at once.