25.2.5 Arc Length and Area of a Sector
Core ideaCut a slice of pizza: arc length = radius × angle (in radians), area = half of 'radius × arc length', and the formulas come out clean precisely because of radians.
Module goal. We break free of the 'angles only run from 0° to 90°' limit, letting angles spin any number of turns and carry a sign, and we bring in radians — a unit better suited to functions — to set the stage for the unit-circle definition.