Closure of Real Interval is Closed Real Interval/Proof 1

Proof
There are four cases to cover:


 * $(1): \quad$ Let $I = \openint a b$.

From Closure of Open Real Interval is Closed Real Interval:
 * $I^- = \closedint a b$


 * $(2): \quad$ Let $I = \hointr a b$.

From Closure of Half-Open Real Interval is Closed Real Interval:
 * $I^- = \closedint a b$


 * $(3): \quad$ Let $I = \hointl a b$.

From Closure of Half-Open Real Interval is Closed Real Interval:
 * $I^- = \closedint a b$


 * $(4): \quad$ Let $I = \closedint a b$.

From Closed Real Interval is Closed in Real Number Line:
 * $I$ is closed in $\R$.

From Set is Closed iff Equals Topological Closure:
 * $I^- = \closedint a b$

Thus all cases are covered.

The result follows by Proof by Cases.