prove that:
A^c ∩ B^c ⊆ (A U B)^c
thank you
Assume $\displaystyle x \in A^-1 \cap B^-1 . $ By Def, $\displaystyle x \notin A $ and $\displaystyle x \notin B$ so $\displaystyle x \in (U-A-B) $ which means $\displaystyle x \in U - (A + B) = (A \cup B)^{-1} $ remember the distributive laws work on sets.