a含有元素1,另外还可含有{2,3,4,5}的任一真子集的元素.而{2,3,4,5}的真子集个数为2^4-1=15所以有15个这样的a:{1},{1,2},{1,3},{1,4},{1,5}{1,2,3},{1,2,4},{1,2,5},{1,3,4},{1,3,5},{1,4,5}{1,2,3,4},{1,2,3,5},{1,2...