I figured that it didn’t have to be exact in this case 😉
If you know some shapes of basic curves in math, than you can easily approximate what you want; like sine/cosine, tan, quadratic equations,…
This is also usually used to calculate Fresnel (with a fifth power); most famous in CG is Schlick’s approximation for the Fresnel curve, but the guy (Schlick) has done more in this area, basically making easier to calculate approximations of difficult functions.
