Unit circle
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A unit circle is a circle whose radius has length 1.
In the Cartesian coordinate system, an equation of the form 
defines a unit circle with center 
.
Trigonometry
An unit circle centered at the origin can be used to calculate values for the basic trigonometric functions. Suppose we draw a ray starting from the origin and meeting the positive x-axis with an angle of 
. If we drop a perpendicular from the point of intersection between the ray and the circle, we have a right triangle with hypotenuse of 
.
Using the definitions 
and 
, we find that 
and 
.
We can read off values for sine and cosine of an angle this way; we can draw the angle and approximate the x and y coordinates of the intersection.
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