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充要条件是重要的数学概念，它主要讨论命题的条件和结论的关系，是理解、掌握一个命题的题设和结论的关系以及一个命题与其逆命题之间的关系的重要工具，用处极其广泛．但教材却把它安排在解析几何的第二章，按教学进度已是在高二的后半阶段了．笔者多年前就提出把“充要条件
The necessary and sufficient condition is an important mathematical concept. It mainly discusses the relationship between the conditions of the proposition and the conclusion. It is an important tool for understanding and mastering the relationship between a topic and the conclusion of a proposition and the relationship between a proposition and its inverse proposition. It is extremely useful. . However, the teaching material has arranged it in the second chapter of analytic geometry, and the progress of teaching is already in the second half of high school. The author put forward the necessary and sufficient conditions many years ago