Can't prove generalized De Morgan's Law

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SUMMARY

The discussion centers on proving generalized De Morgan's Law for a family of subsets indexed by a non-empty set B. The law states that the complement of the union of subsets is equal to the intersection of their complements: (\cup a\in B Sa)c = \bigcap a\in B Sac. The user attempted to demonstrate this by showing mutual subset relationships, successfully establishing that if an element is in the intersection of the complements, it is not in the union, and vice versa.

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Homework Statement


Let B be a non-empty set, and supose that {Sa : a\inB} is an B- indexed family of subsets of a set S. Then we have,
(\cup a\in B Sa)c = \bigcap<sub>a\in B</sub> Sac.


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The Attempt at a Solution


I tried to show that the two were both subsets of each other, but I'm not sure how to do that.
 
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Suppose a\in \bigcap S_\alpha^c. Then a is in all the sets S_\alpha ^c, and so it is not contained in any of S_\alpha. Therefore, it is not contained in their union (by definition). It is therefore contained in union's complement.

Suppose a is in union's complement. No S_\alpha contains a, so all the S_\alpha^c's contain a. Therefore, a is in their intersection.
 

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