Assume that $a_n + a_{n-1} + \cdots + a_0 = 0$. We want to show that $f(1) = 0$. If this is true, then by the Factor Theorem, $x-1$ is a factor of $f(x)$. We have:
$$f(1) = a_n(1)^n + a_{n-1}(1)^{n-1} + \cdots + a_0 = a_n + a_{n-1} + \cdots + a_0 = 0$$
So,
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