To find the domain from the analytical formula, we must first identify the independent variable, then investigate the position of the independent variable, and the position determines the range of the independent variable. Finally, the problem of finding the domain is transformed into the problem of solving the inequality group. Of course, in practical problems, the definition domain of a function is not only limited by analytical expressions, but also by practical meanings, such as time variables generally taking non-negative numbers and so on.

Take your title as an example. Because the analytic expression 2-|x| of the independent variable X is under the quadratic root sign, it must be non-negative, that is, 2-| x | >; 0, so | x |