列方程解应用题时 ,找出相等关系是十分关键的一步 .而在所给问题中 ,相等关系往往不直接给出 ,需要我们去找 .这就要求我们善于从变中找不变 ,进而抓住“不变量”建立 .现以九年义务教育初中《代数》第一册 (上 )第2 2 2页第 16题为例加以说明 .题目　一队学生去校外参加劳动 When solving a problem in a list of equations, it is a crucial step to find out the equal relationship. In the given problem, the equivalence is often not directly given, and we need to find it. This requires us to be good at finding change from change, and then Grasp the establishment of “invariant”. Now take the 16th title of the second-year compulsory education junior high school “Algebra” Volume 1 (top) page 223 as an example to explain. Title 1 Team students go to school to participate in labor