"Let ( ) be a metric space where d(x,y) = |y-x|.

Let be an isometry from the metric space to itself.

Consider the image . Show that for every x in , the image equals or .

Next, show that assuming is continuous, either equals for every x, or it equals for every x."

First, I don't think I understand what "Consider the image " means. I guess it makes sense that each equals or , but I don't know how to prove it.

For the second statement, I know continuity means that for every there is a so when and that it's continuous for each x, but I don't know how to apply this definition to show that it is always either or for every x.

I really appreciate any help, even (or especially!) if it's just explaining very basic things that I probably don't understand correctly. Thanks.