“充要条件”是一个重要的数学概念,对进一步学习数学用处很大,是高等数学论证中最常用的术语,也是统编高中数学第二册教学中公认的难点。中学生学习它有三难:难听懂、难 “The necessary and sufficient conditions” is an important mathematical concept. It is of great use for further study of mathematics. It is the most commonly used term in higher mathematics argumentation, and it is also a recognized difficulty in the compilation of the second volume of high school mathematics teaching. There are three difficulties for middle school students to learn: difficult to understand and difficult