# 1: The Trigonometric Functions

- Page ID
- 7096

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- 1.1: The Unit Circle
- Familiar functions like polynomials and exponential functions don’t exhibit periodic behavior, so we turn to the trigonometric functions. Before we can define these functions, however, we need a way to introduce periodicity. We do so in a manner similar to the thought experiment, but we also use mathematical objects and equations. The primary tool is something called the wrapping function. Instead of using any circle, we will use the so-called unit circle.

- 1.2: The Cosine and Sine Functions
- We started our study of trigonometry by learning about the unit circle, how to wrap the number line around the unit circle, and how to construct arcs on the unit circle. We are now able to use these ideas to define the two major circular, or trigonometric, functions: sine and cosine. These circular functions will allow us to model periodic phenomena such as tides, the amount of sunlight during the days of the year, orbits of planets, and many others.

- 1.3: Arcs, Angles, and Calculators
- An angle is formed by rotating a ray about its endpoint. The ray in its initial position is called the initial side of the angle, and the position of the ray after it has been rotated is called the terminal side of the ray. The endpoint of the ray is called the vertex of the angle. When the vertex of an angle is at the origin in the xy-plane and the initial side lies along the positive x-axis, we see that the angle is in standard position.

- 1.4: Velocity and Angular Velocity
- The connection between an arc on a circle and the angle it subtends measured in radians allows us to define quantities related to motion on a circle. Objects traveling along circular paths exhibit two types of velocity: linear and angular velocity.

- 1.6: Other Trigonometric Functions
- We defined the cosine and sine functions as the coordinates of the terminal points of arcs on the unit circle. As we will see later, the sine and cosine give relations for certain sides and angles of right triangles. It will be useful to be able to relate different sides and angles in right triangles, and we need other circular functions to do that. We obtain these other circular functions – tangent, cotangent, secant, and cosecant – by combining the cosine and sine together in various ways.

*Thumbnail: For some problems it may help to remember that when a right triangle has a hypotenuse of length \(r\) and an acute angle \(θ\), as in the picture below, the adjacent side will have length \(r\cos θ\) and the opposite side will have length \( r\ sin θ\). You can think of those lengths as the horizontal and vertical "components'' of the hypotenuse. Image used with permission (GNU FDL; Michael Corral).*