What non-programming-specific formulas do you use the most whilst working as a programmer?
I'm thinking of non-trivial formulas/theorems from the field of mathematics, logic or science that you constantly put in to practice. The type of formulas they would make you rote learn at uni and that appear in bold in textbooks, rather than lengthy algorithms.
Let us know why the formula comes in handy in your job.
I would say a theorem that even beginner programmers should be aware of is De Morgan's Law:
not (P and Q) = (not P) or (not Q) not (P or Q) = (not P) and (not Q)