A unit circle is a circle with a radius of 1.^{[1]} The unit circle is used to define the sine and cosine functions.
Figure 1: Unit Circle 
In right triangle trigonometry, sine is defined as opposite/hypotenuse, and cosine is defined as adjacent/hypotenuse. Looking at the unit circle, the radius is the hypotenuse of a right triangle. The red leg of the right triangle (see manipulative 1) is the adjacent leg. So the cosine is equal to adjacent/hypotenuse. But, in the unit circle, the hypotenuse is always 1. So the cosine is always equal to the length of the adjacent leg of the right triangle. By similar argument, the sine is always equal to the length of the blue leg, the opposite leg.
Click on the blue point and drag it to change the manipulative.
Manipulative 1: Relationship between unit circle and trigonometric functions.
Created with GeoGebra 

The circle trigonometric identities are equations that can be used to define trigonometric functions. Given a circle with a center at the origin, the trigonometric functions are defined using the x coordinate of a point on the circle (x), the y coordinate of a point on the circle (y), the radius of the circle (r), and the angle of rotation from an intersection of the circle and the xaxis(θ).

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