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Suppose $A \subseteq C$ and $B$ and $C$ are disjoint. Prove that $x \in A \rightarrow x \notin B$.

Basically I need to prove this.

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    Not much to it: Since $A\subset C$, $a\in A\Rightarrow a\in C$. Since $C\cap B=\emptyset$, $a\in C\Rightarrow a\notin B$.2012-01-30
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    Since $x\in A$ and $A\subseteq C$ then ...; but $B$ and $C$ are disjoint, so ...2012-01-30

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