Question Number 47712 by Rio Michael last updated on 13/Nov/18
$${show}\:{that}\: \\ $$$$\:\:^{\:^{\mathrm{2}} \:{C}_{\mathrm{2}} \:} {C}_{{n}} =\:\frac{\mathrm{1}}{\left(\mathrm{1}−{n}\right)!\left({n}−\mathrm{1}\right)\left({n}−\mathrm{2}\right)\left({n}−\mathrm{3}\right)…\mathrm{3}\left(\mathrm{2}\right)\left(\mathrm{1}\right)} \\ $$