“换元法”是指对结构比较复杂、量与量之间的关系不甚明了的命题,通过引入新变量代换原命题的某些量或式子,简化原有结构,使之变成熟悉的、便于研究的命题形式的一种数学方法,但在实际操作过程,经常容易忽视换元前后的等价性而造成错误。下面举几例简要说明: “Redemption method ” refers to the structure is more complex, the relationship between quantity and quantity is not clear proposition, through the introduction of new variables to replace some of the original proposition or formula, simplify the original structure, so that It becomes a kind of mathematic method which is familiar and easy to study propositional form. However, in practice, it is often easy to ignore the equivalence before and after the commutation and cause errors. Here are a few brief instructions: