Subset of Bounded Below Set is Bounded Below

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

Let $B$ be bounded below.

Then $A$ is also bounded below.

Proof
Let $B$ be bounded below.

Then by definition $B$ has a lower bound $L$.

Hence:
 * $\forall x \in B: x \ge L$

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

That is:
 * $\forall x \in A: x \ge L$

Hence, by definition, $A$ is bounded below by $L$.