# Simplification of arctan2(f1,f2) expressions.

Why doesn't `arctan2(sqrt(a^2 + r^2)*sin(phi)*sin(th), sqrt(a^2 + r^2)*cos(phi)*sin(th))`

simplify to phi or at least `arctan2(sin(phi),cos(phi))`

?

Why should it ?

I was working out transformations between coordinate systems. As a test I transformed and then untransformed so there should be no change. However I get this term which won't simplify back to phi.