代数式的求值是初中数学中常见的题型之一.对于一类没有直接给出未知数值的代数式求值问题,同学们普遍感到较为困难.那么如何快捷而又简便地求这些代数式的值呢?这里分类说明. Algebraic evaluation is one of the common problems in junior high school mathematics. For a class of algebraic evaluation that does not give an unknown value directly, students generally feel more difficult. So how to find these algebraic values ​​quickly and easily? Here classification instructions.