You probably have an intuitive idea of what a circle is: the shape of a basketball hoop, a wheel or a quarter. You may even remember from high school that the **radius** is any straight line that starts from the center of the circle and ends at its perimeter.

A unit circle is just a circle that has a radius with a length of 1. But often, it comes with some other bells and whistles.

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A unit circle can be used to define right triangle relationships known as sine, cosine and tangent. These relationships describe how angles and sides of a right triangle relate to one another. Say, for example, we have a right triangle with a 30-degree angle, and whose longest side, or hypotenuse, is a length of 7. We can use our predefined right triangle relationships to figure out the lengths of the triangle's remaining two sides.

This branch of math, known as **trigonometry**, has everyday practical applications such as construction, GPS, plumbing, video games, engineering, carpenter work and air flight navigation.

To memorize a standard unit circle, we need to be able to recall three major components:

- Four quadrants
- 16 angles
- (x, y) coordinates for each of the 16 angles, where the radius touches the circle's perimeter

To help us, we are going to recall a trip to the Unit Pizza Palace. Take a few moments to memorize the following until you can recite it without looking:

**4**pizza slices**3**pies for $6**2**square tables**1**, 2, 3

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