Metric Space is Closed in Itself

Theorem
Let $M = \left({A, d}\right)$ be a metric space.

Then $A$ is closed in $M$.

Proof
From Empty Set is Open in Metric Space, $\varnothing$ is open in $M$.

But:
 * $A = \complement_\varnothing \left({A}\right)$

where $\complement_A$ denotes the set complement relative to $A$.

The result follows by definition of closed set.