# Advice? homework: $\forall x,y \in \mathbb{R}, x \in \mathbb{Q} \land y \notin \mathbb{Q} \implies (x + y) \notin \mathbb{Q}$

Looking for tips to prove the homework

$\forall x,y \in \mathbb{R}, x \in \mathbb{Q} \land y \notin \mathbb{Q} \implies (x + y) \notin \mathbb{Q}$

Can I assume the hypothesis and to yield a contradiction assume that $(x + y)$ is rational, or rather, how should I approach it? So far I know the definitions for an integer $n$ being odd, even, divisible by an interger $m$, has a remainder $r$, and the definition of a rational having integers $p, q$ s.t. $q \neq 0$ and $\frac{p}{q} = Q$

Is there some other tool I might find useful for solving this particular problem? How might I use it? Thanks.

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If $x+y$ and $x$ rational, then $-x$ is rational. so $y=(x+y)-x$ is also rational. –  tetori Feb 5 '13 at 5:12

An approach by contradiction is a good idea. Suppose $x+y\in\mathbb{Q}$. What happens when you subtract $x$? Recall or prove that $\mathbb{Q}$ is closed under addition (and hence subtraction).

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I guess if a rational plus a rational is a rational, then assuming x + y is rational means (x + y) - x is also rational. Therefore leading to the conclusion that y is also rational, contradicting the hypothesis. –  Leonardo Feb 5 '13 at 5:16
@Leonardo Exactly. –  Ben West Feb 5 '13 at 5:17