In addition, it is necessary to distinguish between an empty set (there are no elements in the set, not even 0) and {0} (there are elements in this set, which is 0);
Finally, we should be clear about three operations of the set:
Cross, take a set composed of common elements in two sets;
And, take all the elements in the two sets and remove the duplicate elements to form a set.
The complement set of A is a set, which consists of all the elements removed from the complete set U in set A and the remaining elements.
If element a is an element in the intersection of sets a and b, then a is an element in sets a and b;
If element A is an element in the union of sets A and B, it may be an element in set A, an element in set B, or an element contained in both sets A and B..
If element a is the complement of set a, then element a must not be in set a, but in a complete set;