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列方程解应用题,设未知数比较关键,在初中阶段,一般有三种未知数设法,即设直接未知数、间接未知数、辅助未知数.直接未知数容易设出,多数题目都采取此种设法,也是最常用的;间接未知数往往在设直接未知数不容易列出方程时应用,通过设间接未知数,使之能容易地列出方程,再通过间接未知数求出结果;设辅助未知数往往是在设出直接未知数后还缺少列方程的条件时应用,从而达到列出方程的目的,而辅助未知数在解方程的过程中能够消去,不影响题目的结果.下面就这三种未知数设法,通过例题加以说明.
Solve equations to solve the problem, set the unknown is more critical, in the junior high school stage, there are generally three kinds of unknowns to manage, namely set direct unknowns, indirect unknowns, auxiliary unknowns. Direct unknowns are easy to set, most of the questions are taken this way, is also the most commonly used Indirect unknowns are often applied when the direct unknowns are not easy to list equations. By setting indirect unknowns, it is easy to list the equations, and then find the result through indirect unknowns; set the auxiliary unknowns often after setting the direct unknowns. When the condition of the missing column equation is applied, the purpose of listing the equations can be achieved, and the auxiliary unknowns can be eliminated in the process of solving the equations without affecting the results of the questions. The following three kinds of unknowns are managed and illustrated by examples.