1, set as a real-to-real function, if f(-x)=-f(x), then f(x) is odd function.

2. The 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 (pronounced as "A and B").

3. If a is any real number, the domain of the function is all real numbers greater than 0.

4. In two sets A and B, if any element of set A is an element of set B and any element of set B is an element of set A, we say that set A is equal to set B, that is, A = B..

5. Vector: It has both size and direction.