MHB Saint's question from Yahoo Answers regarding set theory

AI Thread Summary
The discussion addresses a set theory question asking to prove the equation A - (B - C) = (A - B) ∪ (A ∩ C) for any three sets A, B, and C. The proof utilizes four key set theory facts, including the properties of complements and intersections. By applying these properties, the equation is derived step-by-step, demonstrating that the left-hand side simplifies to the right-hand side. The conclusion confirms the validity of the equation through logical set operations. This explanation provides a clear understanding of the relationship between the sets involved.
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Hello Saint,

We will need to use four facts. If $A,B,C\subseteq X$ are three sets, then

1. $(A^c)^c = A$
2. $A-B=A\cap B^c$
3. $(A\cap B)^c = A^c\cup B^c$
4. $A\cap(B\cup C) = (A\cap B)\cup(A\cap C)$

Hence, if $A,B,C\subseteq X$, we see that

\[\begin{aligned}A-(B-C) &= A\cap(B-C)^c\\ &= A\cap(B\cap C^c)^c\\ &= A\cap(B^c\cup (C^c)^c)\\ &= A\cap(B^c\cup C) \\ &= (A\cap B^c)\cup(A\cap C) \\ &= (A-B)\cup(A\cap C).\end{aligned}\]

Therefore, $A-(B-C) = (A-B)\cup(A\cap C)$.

I hope this makes sense!
 
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