Algebra & Polynomials
Limits
Number Theory
Combinatorics
Geometry
Probability
Sequences & Series
Chinese Remainder Theorem: Let m be relatively prime to n. Then each residue class mod m is equal to the intersection of a unique residue class mod n and a unique residue class mod , and the intersection of each residue class mod with a residue class mod m is a residue class mod mn. This means that if we have b ≡ c (mod mn), we can deduce that b ≡ c (mod m) and b ≡ c (mod n). In other words, suppose you wish to find the least number x which leaves a remainder of y₁ when divided by d₁, y₂ when divided by d₂, y₃ when divided by d₃, ... yₙ when divided by dₙ such that d₁, d₂, d₃, ... , dₙ are all relatively prime. Let M = d₁,d₂,d₃,...,dₙ and bi = M/di. If the numbers ai satisfy ai * bi ≡ 1 mod di for every 1 ≤ i ≤ n, then a solution for x is sigma (n, i = 1) (ai * bi * yi mod n).
Euler's Totient Theorem: Let φ(n) be Euler's totient function. If n is a positive integer, φ(n) is the amount of integers in the set {1, 2, 3, ..., n} that are relatively prime to n. If a is an integer and m is a positive integer relatively prime to a, then
a ^ φ(m) ≡ 1 (mod m).
Pascal's Identity: n choose k = (n - 1 choose k - 1) + (n - 1 choose k)
Law of Cosines: For a triangle with edges of length a, b, and c opposites angles of measure A, B, and C, respectively, the Law of Cosines states:
c² = a² + b² - 2ab cos C
b² = a² + c² - 2bc cos B
a² = b² + c² - 2bc cos A