For two sets A and B, if any element in set A is an element in set B, we say that these two sets have an inclusion relationship, and call set A a subset of set B. ..

Symbol:

Included in the symbol: A is included in B-then A is a subset of B or equal to B..

Is the inclusion symbol: a contains B- then b is a subset of a or equal to a.

True inclusion: A is really included in the proper subset of B-if B = {1, 2}, then A={ 1} or {2} or an empty set.

If any element of set A is an element of set B (any a∈A is a∈B), then set A is called a subset of set B, and is denoted as A? B or? b？ A, read as "set A is contained in set B" or set B contains set A ".

Namely:? A∈A has a∈B, what about A? B.?

Proper subset: If the set A is a subset of B, and A≠B, that is, at least one element in B does not belong to A, then A is the proper subset of B, which can be written as: A? B.

Symbolic language: if? A∈A, there are all a∈B, x∈B makes X? A, then a? B.