代数式的求值问题能用构造方程法获得优解，其关键是如何根据题目的特点构造一元二次方程．本文举例介绍几种方法．一、主无法从多个未知数中选一个为主元，把已知等式整理成关于主元的二次方程．例1已知a、入c、d为非零实数，且满足n2／上0／。1＼。。2。oA｛n。。＼／＿n卞LV Algebraic equations can be solved by the construction equation method. The key is how to construct a quadratic equation according to the characteristics of the problem. This article gives an example of several methods. 1. The master cannot choose one principal element from multiple unknowns, and the known equations are organized into quadratic equations about the principal element. Example 1 knows that a, c, and d are non-zero real numbers, and satisfy n2/up 0/. 1. . 2. oA{n. . /_n卞LV