Set Operations (A Wiki Rip)
There are ways to construct new sets from existing ones. Two sets can be “added” together. The union of A and B, denoted by A ∪ B, is the set of all things which are members of eitherA or B.
Examples:

 {1, 2} ∪ {red, white} = {1, 2, red, white}.
 {1, 2, green} ∪ {red, white, green} = {1, 2, red, white, green}.
 {1, 2} ∪ {1, 2} = {1, 2}.
Some basic properties of unions are:

 A ∪ B = B ∪ A.
 A ∪ (B ∪ C) = (A ∪ B) ∪ C.
 A ⊆ (A ∪ B).
 A ∪ A = A.
 A ∪ ∅ = A.
 A ⊆ B if and only if A ∪ B = B.
[edit]Intersections
A new set can also be constructed by determining which members two sets have “in common”. The intersection of A and B, denoted by A ∩ B, is the set of all things which are members of both A and B. If A ∩ B = ∅, then A and B are said to be disjoint.
Examples:

 {1, 2} ∩ {red, white} = ∅.
 {1, 2, green} ∩ {red, white, green} = {green}.
 {1, 2} ∩ {1, 2} = {1, 2}.
Some basic properties of intersections:

 A ∩ B = B ∩ A.
 A ∩ (B ∩ C) = (A ∩ B) ∩ C.
 A ∩ B ⊆ A.
 A ∩ A = A.
 A ∩ ∅ = ∅.
 A ⊆ B if and only if A ∩ B = A.
[edit]Complements
Two sets can also be “subtracted”. The relative complement of A in B (also called the set theoretic difference of B and A), denoted by B \A, (or B −A) is the set of all elements which are members of B, but not members of A. Note that it is valid to “subtract” members of a set that are not in the set, such as removing the element green from the set {1, 2, 3}; doing so has no effect.
In certain settings all sets under discussion are considered to be subsets of a given universal set U. In such cases, U \ A is called the absolute complement or simply complement of A, and is denoted by A′.
Examples:

 {1, 2} \ {red, white} = {1, 2}.
 {1, 2, green} \ {red, white, green} = {1, 2}.
 {1, 2} \ {1, 2} = ∅.
 {1, 2, 3, 4} \ {1, 3} = {2, 4}.
 If U is the set of integers, E is the set of even integers, and O is the set of odd integers, then the complement of E in U is O, or equivalently, E′ = O.
Some basic properties of complements:

 A ∪ A′ = U.
 A ∩ A′ = ∅.
 (A′)′ = A.
 A \ A = ∅.
 U′ = ∅ and ∅′ = U.
 A \ B = A ∩ B′.
Cartesian product
A new set can be constructed by associating every element of one set with every element of another set. The Cartesian product of two sets A and B, denoted by A × B is the set of all ordered pairs (a, b) such that a is a member of A and b is a member of B.
Examples:
 {1, 2} × {red, white} = {(1, red), (1, white), (2, red), (2, white)}.
 {1, 2, green} × {red, white, green} = {(1, red), (1, white), (1, green), (2, red), (2, white), (2, green), (green, red), (green, white), (green, green)}.
 {1, 2} × {1, 2} = {(1, 1), (1, 2), (2, 1), (2, 2)}.
Some basic properties of cartesian products:
 A × ∅ = ∅.
 A × (B ∪ C) = (A × B) ∪ (A × C).
 (A ∪ B) × C = (A × C) ∪ (B × C).
Let A and B be finite sets. Then
  A × B  =  B × A  =  A  ×  B .