→ l12203685 :這題也是李華倫老師的XD 09/27 19:18
※ 引述《WWWZZZXXXMMM (WZXM)》之銘言：
: Let Ａ be a collection of subsets of a set S and Ｂ the collection of
: complementary sets; that is, B 屬於 Ｂ iff S\B 屬於 Ａ.
: (屬於符號打不出來;注意AB之半形全形 有差別)
: (pf):
: a. S\ ∪Ａ = ∩Ｂ
: b. S\ ∩Ａ = ∪Ｂ
: (∪Ａ denotes the union of all sets in Ａ)
a.
S\ ∪Ａ = {x in S: x is not in ∪Ａ}
"x is not in ∪Ａ" means for any A in Ａ ,x is not in A
that is for any B in Ｂ, x is in B
so S\ ∪Ａ ={x in S: x is in ∩Ｂ}
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