{a1,a2}⊆A⊆{a1,a2,a3,a4,a5}求满足上述条件的集合A的个数?
问题描述:
{a1,a2}⊆A⊆{a1,a2,a3,a4,a5}求满足上述条件的集合A的个数?
为什么有8个而不是9个.A={a1,a2}不算吗?
答
分别是{a1,a2},{a1,a2,a3},{a1,a2,a4},{a1,a2,a5}
{a1,a2,a3,a4},{a1,a2,a3,a5},{a1,a3,a4,a5}
{a1,a2,a3,a4,a5}