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。

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};

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};

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};

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};