# 5: Trigonometric Functions of Angles

- Page ID
- 13857

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In the previous chapters, we have explored a variety of functions which could be combined to form a variety of shapes. In this discussion, one common shape has been missing: the circle. We already know certain things about the circle, like how to find area and circumference, and the relationship between radius and diameter, but now, in this chapter, we explore the circle and its unique features that lead us into the rich world of trigonometry.

- 5.1: Circles
- If we wanted to find an equation to represent a circle with a radius of r centered at a point ( h , k ), we notice that the distance between any point ( x , y ) on the circle and the center point is always the same: r.

- 5.2: Angles
- Because many applications involving circles also involve a rotation of the circle, it is natural to introduce a measure for the rotation, or angle, between two rays (line segments) emanating from the center of a circle. The angle measurement you are most likely familiar with is degrees.

- 5.3: Points on Circles Using Sine and Cosine
- While it is convenient to describe the location of a point on a circle using an angle or a distance along the circle, relating this information to the x and y coordinates and the circle equation we explored previously is an important application of trigonometry.

- 5.4: The Other Trigonometric Functions
- In the previous section, we defined the sine and cosine functions as ratios of the sides of a right triangle in a circle. Since the triangle has 3 sides there are 6 possible combinations of ratios. While the sine and cosine are the two prominent ratios that can be formed, there are four others, and together they define the 6 trigonometric functions.

- 5.5: Right Triangle Trigonometry
- In this section, we return to the triangle, and explore the applications of the trigonometric functions to right triangles where circles may not be involved.

*Thumbnail: Image used with permission (CC BY; Openstax)*