举几个例子来推导就可以了.
如1：求{0,1}的子集和真子集.
子集有：{0},{1},{0,1},φ,此时子集个数是2^n(n是元素个数)
真子集有：{0},{1},φ.真子集个数是子集少一个：2^n-1
例2：求{0,1,2}的子集和真子集.
子集有：{0},{1},{2},
{0,1},{0,2},{1,2},
{0,1,2},φ (2^3=8)
真子集：{0},{1},{2},
{0,1},{0,2},{1,2},φ (2^3-1=7)