For some practical problems, the range of independent variables in the function is unknown, which depends on our own analysis. When determining the range of independent variables, we must ensure that practical problems are meaningful within this range. For example, in the example of line 1-6 on page 92 of the textbook, when the independent variable r takes a positive value, the actual questioner is meaningless. Therefore, the range of independent variable r is positive. In some practical problems, functions can be expressed by analytic expressions, so any value that makes analytic expressions meaningless will certainly not make practical problems meaningful, so it must be outside the scope of independent variables.