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