A∖B=B∖A<=>A=B can we actually prove it without using contradiction?

Jaylyn Gibson

Jaylyn Gibson

Answered question

2022-08-06

A B = B A A = B
can we actually prove it without using contradiction?

Answer & Explanation

Howard Robertson

Howard Robertson

Beginner2022-08-07Added 6 answers

Suppose that A B = B A. Since B A B, this implies that A B B. But ( A B ) B = , so A B = , and it follows immediately that A B. Similarly, B A, so A = B.
if A B, then A B B A. If A B, then without loss of generality there is some x A B that is not in B A. But then x A and x B, so A B. If you began by assuming that A = B, this is a proof by contradiction, but that assumption is unnecessary: without it you have a direct proof of the contrapositive of the original implication.

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