sin, cos, and atan2 are "primitive" functions.

Well, once you have a routine for sin, then cos is the same thing, but pi/2 sooner. And I'm fairly sure that's the way it's implemented in the libraries.

Using atan2 to do an inverse sin or cos is a bit of a walk in the park, but essentially I guess you have to find the 'Y' value once you have the 'X', then submit both to atan2, and that will give you the angle you're looking for.

That's a nice brain exercise right after lunch.