The question (from my exam) quoted verbatim:
Show that the additive group of $\mathbb{Z}[x]$ is isomorphic to the group of positive rational numbers under multiplication.
First of all, is this $\mathbb{Z}[x]$ the same as $\mathbb{Z}_x$ (addition modulo $x$)?
Secondly, how do I still get the isomorphism?
Thanks