集合{3，-6,9，-12，15，-18，21}的真子集列举
1、集合A为7个元素，列举该集合的全部子集。
2、含1个元素的子集，此时有7个子集，具体列举如下：
A3={3}，A4={-6}，A5={9};
A6={-12}，A7={15}，A8={-18};
A9={21}；
3、有2个元素的子集情况，此时个数为C(7,2)=21个，
A10={3,-6}，A11={3,9}，A12={3,-12};
A13={3,15}，A14={3,-18}，A15={3,21};
A16={-6,9}，A17={-6,-12}，A18={-6,15};
A19={-6,-18}，A20={-6,21}，A21={9,-12};
A22={9,15}，A23={9,-18}，A24={9,21};
A25={-12,15}，A26={-12,-18}，A27={-12,21};
A28={15,-18}，A29={15,21}，A30={-18,21};
4、此时个数为C(7,3)=35个，列举如下：
A31={3,-6,9}，A32={3,-6,-12}，A33={3,-6,15};
A34={3,-6,-18}，A35={3,-6,21}，A36={3,9,-12};
A37={3,9,15}，A38={3,9,-18}，A39={3,9,21};
5、A40={3,-12,15}，A41={3,-12,-18}，A42={3,-12,21};
A43={3,15,-18}，A44={3,15,21}，A45={3,-18,21};
A46={-6,9,-12}，A47={-6,9,15}，A48={-6,9,-18};
6、A49={-6,9,21}，A50={-6,-12,15}，A51={-6,-12,-18};
A52={-6,-12,21}，A53={-6,15,-18}，A54={-6,15,21};
A55={-6,-18,21}，A56={9,-12,15}，A57={9,-12,-18};
A58={9,-12,21}，A59={9,15,-18}，A60={9,15,21};
A61={9,-18,21}，A62={-12,15,-18}，A63={-12,15,21};
A64={-12,-18,21}, A65={15,-18,21};
7、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={3,-6,9,-12}，A67={3,-6,9,15};
A68={3,-6,9,-18}, A69={3,-6,9,21};
A70={3,-6,-12,15}，A71={3,-6,-12,-18};
A72={3,-6,-12,21}，A73={3,-6,15,-18};
8、A74={3,-6,15,21}, A75={3,-6,-18,21};
A76={3,9,-12,15}, A77={3,9,-12,-18};
A78={3,9,-12,21}, A79={3,9,15,-18};
A80={3,9,15,21}, A81={3,9,-18,21};
9、A82={3,-12,15,-18}, A83={3,-12,15,21};
A84={3,-12,-18,21}, A85={3,15,-18,21};
A86={-6,9,-12,15}, A87={-6,9,-12,-18};
A88={-6,9,-12,21}, A89={-6,9,15,-18};
A90={-6,9,15,21}, A91={-6,9,-18,21};
10、A92={-6,-12,15,-18}, A93={-6,-12,15,21};
A94={-6,-12,-18,21}, A95={-6,15,-18,21};
A96={9,-12,15,-18}, A97={9,-12,15,21};
A98={9,-12,-18,21}, A99={9,15,-18,21};
A100={-12,15,-18,21};
11、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={3,-6,9,-12,15}, A102={3,-6,9,-12,-18};
A103={3,-6,9,-12,21}, A104={3,-6,9,15,-18};
A105={3,-6,9,15,21}, A106={3,-6,9,-18,21};
A107={3,-6,-12,15,-18}, A108={3,-6,-12,15,21};
A109={3,-6,-12,15,21}, A110={3,-6,15,-18,21};
12、A111={3,9,-12,15,-18}, A112={3,9,-12,15,21};
A113={3,9,-12,-18,21}, A114={3,9,15,-18,21};
A115={3,-12,15,-18,21}, A116={-6,9,-12,15,-18};
A117={-6,9,-12,15,21}, A118={-6,9,-12,-18,21};
A119={-6,9,15,-18,21}, A120={-6,-12,15,-18,21};
A121={9,-12,15,-18,21};
13、A122={3,-6,9,-12,15,-18};
A123={3,-6,9,-12,15,21};
A124={3,-6,9,-12,-18,21};
A125={3,-6,9,15,-18,21};
A126={3,-6,-12,15,-18,21};
A127={3,9,-12,15,-18,21};
A128={-6,9,-12,15,-18,21}。
