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

• 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.