25.7.2 The Law of Cosines
Core ideaThis is the Pythagorean theorem leveled up: when the angle isn't a right angle, c²=a²+b²-2ab·cosC adds the correction for being 'slanted'.
Module goal. We extend trig functions from the right triangle to any triangle, using the two great laws to convert freely between sides and angles, wrapping up this stage and handing the next, Sequences, a foundation of measurable relationships.