Let $A,B$ and $U$ be sets so that $A\subseteq U$ and $B \subseteq U$. Prove that $A \subseteq B$ iff $(U\setminus B) \subseteq (U\setminus A).$
Let $A,B$ and $U$ be sets so that $A\subseteq U$ and $B\subseteq U$. Prove that $A\subseteq B$ iff $(U\setminus B)\subseteq(U\setminus A)$.