Basic concepts of set operation:
1. Union: A general set consisting of all elements belonging to set A or set B is called the union of sets A and B, and marked as A ∪ B. ..
2. Intersection: generally, there is a set consisting of all elements belonging to set A and set B, which is called the intersection of set A and set B, and is marked as A ∩ B. ..
3. Complete works: Generally speaking, if a set contains all the elements involved in the problem we are studying, it is called complete works, usually denoted as U.
4. Complement set: Set A, which consists of all elements in complete set U that do not belong to set A, is called the complement set of set A relative to complete set U, or the complement set of set A for short.