The cosine of an angle is the x-coordinate of the point where the angle's
terminal side intersects the Unit Circle.
The inverse cosine is the inverse relation: given an x-coordinate (or
cosine of an angle), find the angle. Values of the
inverse cosine function at special angles can be figured from the picture above:
just take the x-coordinate of the point
the angle goes through.
For example: 
The situation is
similar for the inverse sine function, but now the y-coordinate is used rather
than the x. We have:
