Distance from Subset of Real Numbers to Supremum/Proof 2
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Theorem
Let $S$ be a subset of the set of real numbers $\R$.
Let $x \in \R$ be a real number.
Let $\map d {x, S}$ be the distance between $x$ and $S$.
Let $S$ be bounded above such that $\xi = \sup S$.
Then:
- $\map d {\xi, S} = 0$
Proof
Recall from Real Number Line is Metric Space that the set of real numbers $\R$ with the distance function $d$ is a metric space.
The result is then seen to be an example of Distance from Subset to Supremum.
$\blacksquare$