在众多的条件分式的求值的题目中,有一类题目可用代换方法来求解,本文分类例说如下. 一、主元代换把条件等式中某一个未知数(元)视为常数,解出其余未知数(主元),再代入求值. Among the problems of the evaluation of numerous conditional fractions, there is a class of problems that can be solved by means of substitution methods. The examples of classification in this paper are as follows. 1. The principal meta-replacement treats an unknown (element) in the conditional equation as a constant, Solve the remaining unknowns (primary elements) and substitute them for evaluation.