---
source: knowledge_base
topic: discrete_math
class_level: class_11
difficulty: advanced
---

Discrete Mathematics:
Permutations: P(n,r)=n!/(n-r)!. Combinations: C(n,r)=n!/(r!*(n-r)!).
GCD(a,b)=GCD(b,a mod b). LCM(a,b)=a*b/GCD(a,b).
Modular: a≡b(mod n) means n divides (a-b).
Fermat's Little: a^(p-1)≡1(mod p) for prime p.
De Morgan: NOT(A AND B)=NOT(A) OR NOT(B).