*f*of \(\mathbb{R}\) in \(\mathbb{R}\) in which all angles of rotation

*x*are made to correspond to the

**x-coordinate**of a point P of the circle.

In the illustration above, cos(*x*) = *h*, which is the x-coordinate of point P.

### Notation

The cosine function is noted as “cos” and the image of an angle of rotation x by this function is written as “cos(*x*)” and is read as “cosine *x*” or “cosine of *x*.”