Variable separation is to arrange an unknown variable and other known variables in an inequality (or equation) on both sides of the inequality (or equal sign) respectively. For example, when x belongs to [1/3,3], there is always ln(x+a)≥x, and the range of a is found. It can be sorted as a ≥ e x-x, which is variable separation. Of course, it is only necessary to make a greater than or equal to the maximum value of the right "function" f (x) = e x-x, [1/3,3]; Therefore, the problem is transformed into the problem of finding the maximum value of the function in the closed interval.