Calculate the limit of $a_n = d_∞(f_n, f_{n+1})$, where $f_n(x) = x/n$ and $f_n : [0, 1] → \mathbb{R}$.

Find a metric space $(X, d)$ in which $A = \mathbb{R}_{++}$ is a closed set. (Challenge: what about an open ball instead?)

Consider the circle $A = \{a ∈ \mathbb{R}² : d₂(a, 0) = 1\}$. What is $∂A$?