The essence of functional relationship is to describe the interdependence between variables in the movement process from the perspective of quantitative analysis. However, in the research process, which quantity is used as the independent variable and which quantity is used as the dependent variable (function) is determined by the specific problem being studied.

　　In high school, we learned the concept of inverse functions, from which we know that when a function is a one-to-one mapping, the dependent variable of this function can be used as the independent variable of a new function, and the independent variable of this function can be used as the new function of dependent variables, we call such two functions inverse functions of each other. Let’s further learn the definition and related properties of inverse functions.

　　The definition of inverse function: Let the domain of function be and the value range be . For any value in the range , at least one value corresponding to can be determined in the domain and satisfy the relationship

.

If is regarded as an independent variable and is regarded as a function, a new function can be determined from the above formula

( or ) ,

This new function is called the inverse of function . The domain of the inverse function is and the value range is . Relative to the inverse function, the function is called a direct function.