Many common phenomena have oscillatory or periodic behavior. To model this behavior requires an understanding of functions that exhibit periodic behavior like sine, cosine, and tangent. These functions are introduced using right triangles in this module, which then lets us explore their algebraic relations.

Sine and cosine are now introduced using the unit circle, which is the circle centered at the origin with radius one. This definition of our key periodic functions extends the definition originally introduced with right triangles.

The most basic periodic functions, sine and cosine, were defined for all real numbers. We now study their quotients and reciprocals. However, care must be taken to ensure we do not divide by zero. In this module, we will complete our catalog of periodic functions

In an effort to simplify the work involving our periodic functions, we introduce common identities. This dramatically increases their usefulness in applications. This module will emphasize the development of a small core of identities that are continually needed and can be used to determine a much larger collection. While the number of identities is small in this module, an understanding of these and how to derive others from them is essential for success as you continue your studies.