[集合]證起來怪怪的
不知道證到哪邊才算真正結束
A包含於B <=> A∩B=A
<-
let w∈A => w∈A∩B =>w∈A and w∈B
then w∈A包含於B
->
C
A=(A∩B )∪(A∩B)
A包含於B : w∈A => w∈B
C
so (A∩B )=ψ => A=(A∩B)
這樣寫會太簡短嗎?
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