列举七个元素集合}的真子集
1、集合A为7个元素，列举该集合的全部子集。
元素最少时，即1个元素也没有，此时有子集A1=∅；
元素最多时，即有7个元素，此时有子集A2=A,即：
A2={2,-5,8,-11,14,-17,20}。
2、含1个元素的子集，此时有7个子集，具体如下：
A3={2}，A4={-5}，A5={8};
A6={-11}，A7={14}，A8={-17};
A9={20}；
3、有2个元素的子集情况，此时个数为C(7,2)=21个，
A10={2,-5}，A11={2,8}，A12={2,-11};
A13={2,14}，A14={2,-17}，A15={2,20};
A16={-5,8}，A17={-5,-11}，A18={-5,14};
A19={-5,-17}，A20={-5,20}，A21={8,-11};
A22={8,14}，A23={8,-17}，A24={8,20};
A25={-11,14}，A26={-11,-17}，A27={-11,20};
A28={14,-17}，A29={14,20}，A30={-17,20};
4、有3个元素的子集情况，此时个数为C(7,3)=35个：
A31={2,-5,8}，A32={2,-5,-11}，A33={2,-5,14};
A34={2,-5,-17}，A35={2,-5,20}，A36={2,8,-11};
A37={2,8,14}，A38={2,8,-17}，A39={2,8,20};
A40={2,-11,14}，A41={2,-11,-17}，A42={2,-11,20};
A43={2,14,-17}，A44={2,14,20}，A45={2,-17,20};
A46={-5,8,-11}，A47={-5,8,14}，A48={-5,8,-17};
5、A49={-5,8,20}，A50={-5,-11,14}，A51={-5,-11,-17};
A52={-5,-11,20}，A53={-5,14,-17}，A54={-5,14,20};
A55={-5,-17,20}，A56={8,-11,14}，A57={8,-11,-17};
A58={8,-11,20}，A59={8,14,-17}，A60={8,14,20};
A61={8,-17,20}，A62={-11,14,-17}，A63={-11,14,20};
A64={-11,-17,20}, A65={14,-17,20};
6、有4个元素的子集情况
此时个数为C(7,4)=C(7,3)=35个，列举如下：
A66={2,-5,8,-11}，A67={2,-5,8,14};
A68={2,-5,8,-17}, A69={2,-5,8,20};
A70={2,-5,-11,14}，A71={2,-5,-11,-17};
A72={2,-5,-11,20}，A73={2,-5,14,-17};
A74={2,-5,14,20}, A75={2,-5,-17,20};
A76={2,8,-11,14}, A77={2,8,-11,-17};
A78={2,8,-11,20}, A79={2,8,14,-17};
A80={2,8,14,20}, A81={2,8,-17,20};
7、
A82={2,-11,14,-17}, A83={2,-11,14,20};
A84={2,-11,-17,20}, A85={2,14,-17,20};
A86={-5,8,-11,14}, A87={-5,8,-11,-17};
A88={-5,8,-11,20}, A89={-5,8,14,-17};
A90={-5,8,14,20}, A91={-5,8,-17,20};
A92={-5,-11,14,-17}, A93={-5,-11,14,20};
A94={-5,-11,-17,20}, A95={-5,14,-17,20};
A96={8,-11,14,-17}, A97={8,-11,14,20};
A98={8,-11,-17,20}, A99={8,14,-17,20};
A100={-11,14,-17,20};
8、有5个元素的子集情况
此时个数为C(7,5)=C(7,2)=21个，列举如下：
A101={2,-5,8,-11,14}, A102={2,-5,8,-11,-17};
A103={2,-5,8,-11,20}, A104={2,-5,8,14,-17};
A105={2,-5,8,14,20}, A106={2,-5,8,-17,20};
A107={2,-5,-11,14,-17}, A108={2,-5,-11,14,20};
A109={2,-5,-11,14,20}, A110={2,-5,14,-17,20};
A111={2,8,-11,14,-17}, A112={2,8,-11,14,20};
9、A113={2,8,-11,-17,20}, A114={2,8,14,-17,20};
A115={2,-11,14,-17,20}, A116={-5,8,-11,14,-17};
A117={-5,8,-11,14,20}, A118={-5,8,-11,-17,20};
A119={-5,8,14,-17,20}, A120={-5,-11,14,-17,20};
A121={8,-11,14,-17,20};
10、有6个元素的子集情况，此时个数为C(7,6)=C(7,1)=7个：
A122={2,-5,8,-11,14,-17};
A123={2,-5,8,-11,14,20};
A124={2,-5,8,-11,-17,20};
A125={2,-5,8,14,-17,20};
A126={2,-5,-11,14,-17,20};
A127={2,8,-11,14,-17,20};
A128={-5,8,-11,14,-17,20}。
