A third class of functions that are prevalent in higher mathematics is the trigonometric functions. For a full discussion of trigonometry, see the SparkNote on Trigonometry. In the following lessons, we'll take a brief look at the trigonometric functions. The trigonometric functions have to do with angles in the coordinate plane. They are unique because the input is an angle measure, and the output is a ratio. The six trigonometric functions are sine, cosine, tangent, cosecant, secant, and cotangent. They are periodic functions--their values repeat at regular intervals. The sine and cosine functions are especially useful in modeling repetitive motion, like oscillation. Sine and cosine can also be thought of as the most important of the trigonometric functions because of their relevance to the unit circle and the fact that the other four trigonometric functions can easily be expressed in terms of sine and cosine. In addition to examining the trigonometric functions and their graphs, we'll discuss the inverse trignometric functions and also some trigonometric equations.