Closed Real Interval/Examples/Example 1

Examples of Closed Real Intervals
Let $I$ be the closed real interval defined as:
 * $I := \closedint 1 3$

Then $3 \in I$.

Proof
By definition of open real interval:
 * $I = \set {x \in \R: 1 \le x \le 3}$

As $3 \le 3$ it follows that $3 \in I$.