P-adic Norm not Complete on Rational Numbers/Proof 2/Lemma 3

Theorem
Let $x_1, p, q \in Z_{\gt 0}$ be any positive integers.

Let $a = x_1^q + p$

Let $f \paren{X} \in \Z [X]$ be the polynomial:
 * $X^q - a$

Then:
 * $\map f {x_1} \equiv 0 \pmod p$