Solution
= Solution
Every point of $A$ differs from $a$, so its singleton is an <open ball> by part (ii). The <union> of these singleton <open sets> is $A$. Thus \b[$A$ is open] whenever $a\notin A$.
= Solution
Every point of $A$ differs from $a$, so its singleton is an <open ball> by part (ii). The <union> of these singleton <open sets> is $A$. Thus \b[$A$ is open] whenever $a\notin A$.