25.3.1 Defining Trig Functions on the Unit Circle
Core ideaOn a circle of radius 1, the y-coordinate where the angle's terminal side meets the circle is sinα, and the x-coordinate is cosα.
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.