I'm guessing you mean for the Cartesian co-ordinate system, what the trigonometric functions are.
Sine is for the y-axis, and Cosine is for the x-axis. You have it the wrong way around.
You can think of the plane as the "top" of a cylinder. The important things on the top can be defined by co-ordinates, or coordinates, where and .
The height of the cylinder is defined by the length on the axis in space. Since this doesn't depend on or , it doesn't depend on or either. Therefore, is just .
So to answer your question
.
These are known as Cylindrical Polar Co-ordinates.
Or it could be the Spherical coordinate system, which if you draw a line 1 unit and move it about the origin, then find where the furthest point is from the origin.
If this is the case, then x is no longer just cosine and y is no longer just sine, because as you move it about (0,0,0) through the z-axis as well, then it will be shorter than just sin(x) and cos(x).