De Morgan's Laws (Set Theory)/Relative Complement/Complement of Union

Theorem
Let $S, T_1, T_2$ be sets such that $T_1, T_2$ are both subsets of $S$.

Then, using the notation of the relative complement:
 * $\complement_S \left({T_1 \cup T_2}\right) = \complement_S \left({T_1}\right) \cap \complement_S \left({T_2}\right)$

Proof
Let $T_1, T_2 \subseteq S$.

Then from Union is Smallest Superset:


 * $T_1 \cup T_2 \subseteq S$

Hence: