The foundation of all science and technology.
An auto race I recently attended also ran a 50/50 raffle, which is where a single winner receives 50% of the money collected from ticket sales.
Gauss famously (and possibly fictionally) computed the sum between 1 and 100 at a young age. His original logic relies on symmetry and geometrical thought, especially triangles.
A summer day in 2019 reading Complex Variables and Applications by Churchill inspired what I now realize is almost certainly a useless (but perhaps slightly amusing) result. Enjoy.