We show the cosine in the unit circle. The unit circle is a circle with radius .

The hypotenuse of the triangle in the unit circle has a length of 1. In this case cosine is equal to the adjacent:

The sine function is the length of the adjacent with regards to the angle . One can easily observe that:

(1)

This is the plot of the cosine function between 0 and . It repeats periodically:

Here are some values of the sine functions that are worth remembering:

Alpha in degree | Alpha in radians | Value of cosine of alpha |
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