Hallo everyone,
Do you know any simple formulas that are often unknown to many? For me, some can be mentioned are
Graph: Euler characteristic, Handshaking theorem, Caley Formula, Kirchhoff's theorem, Kőnig's theorem
Polygon: 2-Ear Theorem, incenter formula of a triangle, pick theorem, Shoelace Formula
Logic: Pigeonhole
Prime: Fermat, Wilson's theorem
Number: McNugget theorem
Fibonacci: Zeckendorf's theorem
Counting: Burnside Lemma, stars and bars
Combinatorics Identity: Vandermonde's Identity, Hockey-stick identity
Math: Linearity of Expectation
Other: harmonic series sum
... And what about you? Can you share some?
Thank you.