2. Operation: Intersection ∪ union CuA is the complement of A, belonging to ∈, subset? , proper subset?

3. Symbols of commonly used number sets:

(1) The set of all non-negative integers is usually called the set of non-negative integers (or the set of natural numbers), and is recorded as n.

(2) Exclude the set of 0 from the set of non-negative integers, also known as the set of positive integers, and record it as N+ (or N*).

(3) The set of all integers is usually called the set of integers, and is denoted as z..

(4) The set of all rational numbers is usually referred to as the rational number set for short, and is recorded as Q..

(5) The set of all real numbers is usually called the set of real numbers, and is denoted as r.

(6) the complex set is c.

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