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The relation $gh = hg$ means this group $G$ is commutative. $\langle g\rangle$ and $\langle h \rangle$ are cyclic subgroups of G. Still have no idea how to conclude $|gh|$ is finite.

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    **Note:** The condition $gh=hg$ does **not** mean that the group $G$ is commutative. It only means that $g$ and $h$ commute. It is perfectly possible for two elements in a noncommutative group to commute.2012-02-13

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Take powers of $gh$. The condition $gh = hg$ will tell you that $(gh)^n = g^n h^n$. If $g^m = e$ the identity, and if $h^n = e$, then we see that $gh^{mn} = g^{mn}h^{mn} = e$.

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    @Ragib: If $g$ and $h$ commute, then $\text{ord}(gh) = \text{lcm}(\text{ord}(g), \text{ord}(h))$, and can be in general, less than $mn$.2012-02-17