Closure of Open Real Interval is Closed Real Interval

Theorem
Let $\struct {\R, \tau_d}$ be the real number line with the usual (Euclidean) topology. Let $\openint a b$ be an open interval of $\R$.

Then the closure of $\openint a b$ is the closed interval $\closedint a b$.

Proof
From Limit Points of Open Real Interval, the limit points of $\openint a b$ consist of:
 * the points $\openint a b$ itself

and
 * the points $a$ and $b$.

By definition, the closure of $\openint a b$ is the union of $\openint a b$ and its limit points.

Hence the result, by definition of the closed interval $\closedint a b$.