Definition: a set of all elements belonging to a or b, called the union of a and B.

A∪B Pronunciation: A and B.

Property: A∪A=A

A ∪ φ = φ ∪ A = A (where φ) mathematically represents an empty set.

A∪B=B∪A

The definition of a set consisting of all elements belonging to set A or set B is called union, marked as A∪B, and pronounced as "A and B".

A∪B={xIx∈A or x∈B}

In other branches of set theory and mathematics, the union of a set is the set of all the elements of these sets, but it does not contain other elements.

This is a crossroads.

Definition: the set consisting of all elements belonging to set A and set B is called the intersection of A and B.

Express: A∩B Pronunciation: A to B.

Properties: A ∩ A = A ∩ φ = φ A ∩ B = B ∩ A?

A∩B∈A，A∩B∈B

(A∩B)∩C=A∩(B∩C)=A∩B∩C

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