This is why I was getting confused because its a written up question not from a text and we were just given the formal definition of an interval from like back in calc.
A subset I of R is an interval provided that for all x, y, z ∈ R, if x < y < z and x ∈ I and z ∈ I then y ∈ I. (Given Def of Interval)
However, when I was looking through previous books they just jumped into open and closed intervals and nothing really about if the set is an interval. For the first part The set[1,3)={x∈R:1≤x<3}is an interval I would say its an interval but I don't know how to prove that it is. And then for the second part For any two intervals I , J , if I ∩ J ̸= ∅ then I ∪ J is an interval I'm not where to even start.
Let’s use that definition.
If we must show that .
If we have either or then we are done because each of is interval. WHY?
So suppose that and . We are given that .
Three cases: i) if we are done. Why?
ii) if because is an interval, so
iii) likewise if because is an interval, so .
We are done.