Closure of Union of Adjacent Open Intervals

Theorem
Let $a, b, c \in R$ where $a < b < c$.

Let $A$ be the union of the two adjacent open intervals:
 * $A := \openint a b \cup \openint b c$

Then:
 * $A^- = \closedint a c$

where:
 * $A^-$ is the closure of $A$.