Cosine
The cosine is per definition the first coordinate on the unit circle.
Explanation
Take a point P (x, y) on the unit circle. Then, per definition, the cosine of θ is the first coordinate of P and the sine is the second coordinate.
In this way obtuse angles can also have a sine or cosine. The name cosine comes from complementary sine. There are only two functions f (x) and g (x) complying with the conditions
and these are the sine and the cosine, as
Example 1
You can see in the picture that cos (0) = cos (2π) = 1 and cos (90°) = cos (½π) = 0 and cos (180°) = cos (π) = −1.