Subset of Bounded Above Set is Bounded Above

Theorem
Let $A$ and $B$ be sets of real numbers such that $A \subseteq B$.

Let $B$ be bounded above.

Then $A$ is also bounded above.

Proof
Let $B$ be bounded above.

Then by definition $B$ has an upper bound $U$.

Hence:
 * $\forall x \in B: x \le U$

But by definition of subset:
 * $\forall x \in A: x \in B$

That is:
 * $\forall x \in A: x \le U$

Hence, by definition, $A$ is bounded above by $U$.