For example: A={ 1, 2, 3, 4} and B={ 1, 2, 3, 4, 5} Then, because four of the five numbers in set B are the same as set A, and there is one more number than set A, then set A is included in set B. 。
Between a number and a set, for example:
3 and A={ 1, 3,5}, because the three numbers in set A have 1, which is the same as the previous numbers, so there is: 3∈ set A.
Some sets have only one number, for example, C={3}
If we give these two numbers: 3, C={3}, because the set is higher than "high", this number can only be subordinate to the set, but to ensure that this number also exists in that set, there is: 3∈ set c.
If it is a relationship between sets, it is contained in (contained in), and if it is a relationship between numbers and sets, it is used to belong to.
In addition, if A={ 1, 2, 3, 4} and B={ 1, 2, 3, 4}, because the numbers in set a and set b are exactly the same, there is: set A= set b.