25.2.4 Converting Between Degrees and Radians
Core idea180° and π are just two ways of naming the same angle, and once you have that yardstick you can switch back and forth freely.
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.