The purpose of this lesson is for students to recall how to determine the value of the cosine, sine, and tangent of an angle for a right triangle. In this lesson, students connect their understanding of right triangle trigonometry with a point on a circle by using the horizontal and vertical coordinates of a point on the circle as the legs of a right triangle and the radius of the circle connecting to that point as the hypotenuse.

This lesson helps prepare students to transition to thinking of cosine and sine as functions of an angle, by establishing how to conceptualize points in Quadrant I as a vertex of a right triangle, where the origin and a point on the -axis are the other vertices and the right angle is on the -axis. Using the structure of the coordinate plane this way, students identify the coordinates of the triangle vertex in Quadrant I as the cosine and sine of the angle at the origin when the hypotenuse is one unit (MP7).