Unit circle
The unit circle is the circle with radius 1, whose center is at the origin (0, 0).
Explanation
The cosine is the x-coordinate of a point P (cos t, sin t) on this circle and the sine is the y-coordinate
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) that meet the following conditions
and that are the sine and the cosine, because
Example 1
In the drawing you can see that cos (0) = cos (2π) = 1 and cos (90°) = cos (½π) = 0 and cos (180°) = cos (π) = −1.
Example 2
In the drawing you can see that sin (0) = sin (π) = sin (2π) = 0 and sin (90°) = sin (½π) = 1.