Solution: In other branches of set theory and mathematics, the union of sets is a set composed of all the elements of these sets, but does not contain other elements.

If A and B are sets, the union of A and B is a set containing all elements of A and all elements of B, but no other elements. The union of a and b is usually written as "A∪B" and pronounced as "a and b", which is expressed in symbolic language, namely: A∪B={x|x∈A, or x∈B}