Proof: A' is a Subset of B iff AUB is Universal Set | Set Theory

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Let A and B be subsets of a universal set U. Then the complement of A, written A', is a subset of B if and only if A union B is the universal set! We'll go over this set theory proof in today's set theory lesson. All we need to do is use our definitions of set complement, set equality, subset, and set union, and we'll be able to prove this biconditional set theory result.

I hope you find this video helpful, and be sure to ask any questions down in the comments!

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The outro music is by a favorite musician of mine named Vallow, who, upon my request, kindly gave me permission to use his music in my outros. I usually put my own music in the outros, but I love Vallow's music, and wanted to share it with those of you watching. Please check out all of his wonderful work.

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+WRATH OF MATH+


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You are always great with your lectures, they are super useful

WhenCatPlayz
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Can you pls do it in Identity base not in definition

Akitchen
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bro you're saving my life, thank you

anonymousvevo
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1/ A is a subset of the universal set S means, by definition of the universal set, A U A' = S. As S = A U B, we should have A U A' = A U B. By definition of set union, this means A' = B, and therefore A' is a subset of B.
2/ On one hand, A and B be are subsets of a universal set S. This implies, by definition of set subset, that A U B is a subset of S.
On the other, if s is an element of the universal set S then, by definition of the universal set, S = A U A' and s is of course an element of A or A'. As A' is a subset of B then s is an element of A or B, that is s is an element of A U B and the universal set S is a subset of A U B. So A U B and S are subsets of each other, which is a way to say A U B = S.

azizhani