Representation methods of 1. set: enumeration method, description method and graphic method are commonly used.

2. union: A∪B={x| x∈A or x∈B}.

3. Number of finite subsets: If the number of elements in set A is n, then A has 2n subsets, 2n- 1 nonempty subset and 2n-2 nonempty proper subset.

4. Description: A method of describing the common attributes of elements in a collection and writing them in braces to represent the collection.

5. The elements in the set must be definite. That is, a set is determined, and it is also determined whether any element is an element of the set.