I think I'm going to put together a DLL library for mathematics calculations that I tend to use or see others use in game development. Things like linear functions (the whole idea was over the fact that I had to go and look up the slope point form... again), parabolas (the aiming a ball in an arc problem), the quadratic formula, and other such problems. My main goal is to make something accessible over multiple languages and engines, mostly. I'm still gathering a list of all the problems I want to add.
As someone who spends most of their dev time looking up math formulas, I second this motion!
I'd love to hear which ones you have to look up repeatedly over time, or ones that tend to be the most problematic.
The ones that pop into mind right now are...
Rotating a vector (2D and 3D)
Projecting point to line and point to plane
Finding shortest angle between two vectors