1. commutative law

A∩B=B∩A

A∪B=B∪A

2. Association law

(A∩B)∩C=A∩(B∩C)

(A∪B)∪C=A∪(B∪C)

3. Distribution law

A∩(B∪C)=(A∪B)∩(A∪C)

A ∪( B∪C)=(A∪B)∪( A∪C)

2 De Morgan's Law

Cs(A∩B)=CsA∪CsB

cs(A∪B)= CsA∪CsB

3 "Exclusion principle"

When we study a set, we will encounter problems about the number of elements in the set. We write the number of elements in a finite set as card (a). For example, A={a, b, c}, then card (A)=3.

Card (A∪B)= Card (A)+ Card (B)- Card (A∪B)

Card (A∪B∪C)= Card (A)+ Card (B)+ Card (C)- Card (A∪B)- Card (C∪A)+ Card (A ∪.

1985 Cantor, a German mathematician and founder of set theory, talked about the word set. Enumeration and description are common methods to represent collections.

Law of absorption

A ∨( A∩B)= A

A∩(A∪B)=A

Supplementary law

A∪CsA=S

a∩CsA =φ