TITLE: Irreducible polynomial over $\mathbb{Q}[x]$ has even degree if the sum of two distinct roots lies in $\mathbb{Q}$
QUESTION [6 upvotes]: Let $f(x) \in \mathbb{Q}[x]$ be an irreducible polynomial and suppose that $\alpha, \beta \in E/\mathbb{Q}$ are two distinct roots of $f(x)$ in its splitting field satisfying $\alpha + \beta \in \mathbb{Q}$. 
How do I show that the degree of $f(x)$ is even?

REPLY [6 votes]: Suppose $\alpha+\beta = r$.
If $\gamma$ is any root of $f(X)$, pick some automorphism $\tau_\gamma$ of the splitting field sending $\alpha$ to $\gamma$.  Then $\gamma + \tau_\gamma(\beta) = r$.
Now partition the roots of $f$ into pairs that add to $r$.