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

cos-circle

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

    equation

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

(1)   equation

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

cos

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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equation equation equation