ข้อเจ็ด
ตอบ ค)
$A\cap (B-C)=(A\cap B)-(A\cap C)$
$(A\cap B)-(A\cap C)=(A\cap B)\cap (A'\cup C')$
$ =(A\cap B\cap A')\cup (A\cap B\cap C')$
$ =\varnothing \cup (A\cap B\cap C')$
$ =A\cap (B-C)$
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