De Morgan's Laws (Set Theory)/Relative Complement/Complement of Union/Proof 1
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Theorem
- $\relcomp S {T_1 \cup T_2} = \relcomp S {T_1} \cap \relcomp S {T_2}$
Proof
Let $T_1, T_2 \subseteq S$.
Then from Union is Smallest Superset:
- $T_1 \cup T_2 \subseteq S$
Hence:
\(\ds \relcomp S {T_1 \cup T_2}\) | \(=\) | \(\ds S \setminus \paren {T_1 \cup T_2}\) | Definition of Relative Complement | |||||||||||
\(\ds \) | \(=\) | \(\ds \paren {S \setminus T_1} \cap \paren {S \setminus T_2}\) | De Morgan's Laws: Difference with Union | |||||||||||
\(\ds \) | \(=\) | \(\ds \relcomp S {T_1} \cap \relcomp S {T_2}\) | Definition of Relative Complement |
$\blacksquare$
Source of Name
This entry was named for Augustus De Morgan.