充要条件是一个重要的数学概念,也是教学中一个难点.在应用时,往往几何、代数、三角多学科内容相互渗透,综合性较强.若用集合理论来表述充要条件,在处理某些问题时,往往思路清晰,结论准确.1　定义定义:设满足条件A的对象集合为M,满足条件B的对象集合为N.1)如果MN,则A为B的充分 The necessary and sufficient condition is an important mathematical concept, and it is also a difficult point in teaching. When applied, the multi-disciplinary contents of geometry, algebra, and trigonometry often infiltrate each other, and the comprehensiveness is strong. If a set theory is used to express sufficient and necessary conditions, some In these problems, the thinking is often clear and the conclusions are accurate. 1 Definition: Set the set of objects satisfying condition A to M, and the set of objects satisfying condition B to N.1) If MN, then A is sufficient for B