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Here is another of my favourite examples:

Prove that $(\R,+)$ ({\mathbb R},+)$ and $(\R[x],+)$ ({\mathbb R}[x],+)$ are isomorphic as abelian groups.

It is fairly easy to prove that they are actually isomorphic as $\Q$-vector {\mathbb Q}$-vector spaces, which is a stronger result; other than that I don't know any way of proving this.

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Here is another of my favourite examples:

Prove that $(\R,+)$ and $(\R[x],+)$ are isomorphic as abelian groups.

It is fairly easy to prove that they are actually isomorphic as $\Q$-vector spaces, which is a stronger result; other than that I don't know any way of proving this.