Several roots in one field extension?

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I'm curious as to why, if $r_1$ is a root of $X^4+8X+12$, then no other root can live inside $\mathbb{Q}(r_1)$. This follows immediately from knowledge of the Galois group of $X^4+8X+12$, but are there other methods (apart from inserting $a_1+a_2r_1+a_3r_1^2+a_4r_1^3$ into the polynomial and setting this zero - I know that the polynomial is irreducible)?