# Verify an equivalence relation.

For a set $E$ of real numbers, define two points in $E$to be rationally equivalent if their difference belongs to $Q$. Prove that this defines an equivalence relation.

(i) is trivial as 0 is a rational number

(ii) Suppose $r - q$ is rational. Then as $s \in Q \implies -s \in Q$, $q-r= -(r-q) \in Q$

How can I do (iii)?

I think you mean $E$ a set of real numbers. $r-q\in \Bbb Q$ and $q-s\in \Bbb Q$, so $r-s=(r-q)+(q-s)\in \Bbb Q$.

• Yes $E$ is a set of real numbers. – Alexander Feb 24 '14 at 13:21