Question Number 90717 by ~blr237~ last updated on 25/Apr/20
$${Prove}\:{that}\:{for}\:{all}\:{integer}\:{r}\geqslant\mathrm{2}\: \\ $$$$\:{HCF}\left({r}^{{n}} −\mathrm{1};{r}^{{m}} −\mathrm{1}\right)={r}^{{HCF}\left({n};{m}\right)} −\mathrm{1} \\ $$