In ancient China, the word "Xin" and the word "Han" were universal and both had the meaning of "Han". Li's definition is: "every formula contains heaven, which is a function of heaven." In ancient China, four words were used to represent four different unknowns or variables: heaven, earth, people and things.
The meaning of this definition is: "Whenever a formula contains a variable X, the formula is called a function of X", so "function" means that the formula contains a variable.
Let the domain of the function f(x) be D, and the interval I is contained in D. If for any two points x 1 and x2 on the interval, when X 1
If for any two points x 1 and x2 on the interval I, when X 1