Separated Sets are Clopen in Union

Theorem
Let $T = \left({S, \tau}\right)$ be a topological space.

Let $A$ and $B$ be separated sets in $T$.

Let $H = A \cup B$ be given the subspace topology.

Then $A$ and $B$ are each both open and closed in $H$.

Proof
By hypothesis, $A$ and $B$ are separated:
 * $A \cap B^- = A^- \cap B = \varnothing$

Then:

Since the intersection of a closed set with a subspace is closed in the subspace, $B$ is closed in $H$.

Since $A = H \setminus B$ and $B$ is closed in $H$, $A$ is open in $H$.

By the same argument with the roles of $A$ and $B$ reversed, $A$ is closed in $H$ and $B$ is open in $H$.

Hence the result.