25.2.3 Radian Measure: Sizing an Angle by Arc Length
Core ideaInstead of degrees, we measure an angle by 'how many radii of arc it sweeps across', so one full turn corresponds to an arc of 2π.
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.