25.3.2 Signs of Trig Values by Quadrant
Core ideaWhich quadrant the point lands in settles whether its x and y are positive or negative, and the signs of sin, cos, and tan follow right along.
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.