Why these points are isolated points in this exercise This is an exercise from a calculus book I'm reading:

I can do the exercise but I don't understand the $(\dots)$ in $(c)$. The $x$ are in $\mathbb R$ and $\mathbb R$ does not have isolated points (an isolated point of a set is a point that is not an accumulation point of the set). Please can someone explain me why these points are isolated?

Isolated in this context means isolated within the set:  that there are not points arbitrarily close.  If I define the set $\{\frac 1n: n \in \Bbb N\}$ each point is isolated because for each point $x_n$ I can find an $\epsilon$ so that there are no other points of the set within $(x_n-\epsilon,x_n+\epsilon)$
