(1) in the process of a change, the value that can take different values is called a variable. A quantity with a constant value is called a constant. Constants and variables are relative. The premise of judging constants and variables is "in a certain changing process", and the same quantity can be constant or variable in different changing processes, depending on the conditions of the problem. Constants and variables are not necessarily quantities, but can also be constants or variables. ② There are two variables X in a certain change process. If y has a unique fixed value corresponding to each fixed value of x within the range of values, then x is an independent variable, y is a function of x, and the function is not a number, which refers to the relationship between two variables in a certain change process. ③ The value of independent variables must make the algebraic expression with independent variables meaningful. The range of independent variables can be infinite or limited. It can be several numbers, or it can be a number, indicating. The value of the independent variable must make the practical problem meaningful. (4) If the independent variable takes a fixed value within the range, the function has a unique fixed value corresponding to it, which is called a function value of the function. When a function is expressed by an analytic expression, the function value is the value of an algebraic expression, and the function value is unique. However, the corresponding independent variable values can be multiple. The range of function values varies with the range of independent variables. ⑤ There are three representations of functions: analytical method, list method and image method. These three representations have their own characteristics and are usually used in combination in applications. The general steps of drawing function images are: list, drawing points and connecting lines.