Here is a more general result.
Theorem: Let be a non-Archimedean normed field with completion . Let be a finite separable extension of degree . Then there exists a degree separable field extension such that .
In particular, as long as the completion of admits a separable field extension of a certain degree , so does itself, necessarily of the form by the primitive element theorem. Moreover, as long as has characteristic zero and carries a nontrivial discrete valuation, it admits finite separable extensions of all finite degrees.
For a proof of this theorem using Krasner's Lemma, see Section 3.5 of
http://math.uga.edu/~pete/8410Chapter3.pdf
When the norm corresponds to discrete valuation (e.g.
the -adic norm for a prime ideal of a number field ) one can get away with less: by weak approximation, there exists with . For any positive integer prime to the characteristic of , by Eisenstein's Criterion the polynomial is (separable and) irreducible even over the completion , so is certainly irreducible over .
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