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Question Number 132977 by Raxreedoroid last updated on 17/Feb/21
$$\mathrm{Show}\:\mathrm{that}: \\ $$$$\:^{{n}} {C}_{{r}} =\frac{\underset{{k}=\mathrm{0}} {\overset{{r}−\mathrm{1}} {\prod}}{n}−{k}}{{r}!} \\ $$