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Feb 29 at 16:11 comment added Sky I have edited something. Pls See the new question.
Feb 29 at 16:08 history edited Sky CC BY-SA 4.0
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Feb 29 at 9:11 history edited YCor CC BY-SA 4.0
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Feb 29 at 8:12 comment added Emil Jeřábek So, the answer is negative for $x=2^{1/3}\omega$, $y=2^{1/3}\overline\omega$. Then any linear combination $z$ is $2^{1/3}$ times an element of $\mathbb Q(\sqrt3i)$, hence $z\sigma(z)$ is a rational multiple of $2^{2/3}$, which is not rational (if nonzero).
Feb 29 at 7:42 comment added Emil Jeřábek $\sigma$ is just complex conjugation. Anyway, the question boils down to: does every $\mathbb Q$-linear subspace of $L$ of dimension $2$ contain a nonzero $z$ such that $z\sigma(z)$ is rational?
Feb 29 at 6:49 history asked Sky CC BY-SA 4.0