计算有限集合中的元素个数问题,在实际问题中应用非常广泛,特别是在各级各类数学竞赛以及后续课程的排列、组合与概率章节中经常用到,因此,了解掌握这部分内容很有必要.对于一个有限集,可以用一条封闭曲线围成平面图形来表示,用这个平面图形的内部来表示它所含全部元素的个数.若这个有限集合记为S, The problem of calculating the number of elements in a finite set is very widely used in practical problems, especially in the chapters of various types of mathematics competitions and the arrangement, combination and probability of subsequent courses. Therefore, it is very important to understand this part of the content. It is necessary that for a finite set, a closed curve can be represented by a plane graphic, and the interior of the plane graphic is used to represent the number of all elements it contains. If this finite set is denoted as S,