Ask HN: What are some neat or useful equations you know?

[y33t]

Something worth saving in a programmable calculator, like Naismith's Rule [1] or calculating the day of week for any date [2].

1: https://en.wikipedia.org/wiki/Naismith%27s_rule

2: https://cs.uwaterloo.ca/~alopez-o/math-faq/node73.html