(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

I can repeat the proof, but then I won't understand it.

How do I prove that an open interval like (1,2) is open?

2. Relevant equations

I know it has something to do with neighbourhood with a given epsilon, but I dont really get what a neighbourhood is, I have a maybe rough picture of it but not sufficient enough

it'd be nice if you could explain in some sort of analogy or plain english :)

3. The attempt at a solution

let x be in (1,2)

|x-1|>=e |x-2|=>e

e=min{x-5,17-x}

then neighbouthood will be inside (1,2)

and I just get lost from there

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# How to prove an interval is open?

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