Question Number 12218 by tawa last updated on 16/Apr/17
$$\mathrm{5}!!\:=\:? \\ $$
Commented by tawa last updated on 16/Apr/17
$$\mathrm{How}\:\mathrm{is}\:\mathrm{the}\:\mathrm{answer}\:\mathrm{15} \\ $$
Commented by prakash jain last updated on 16/Apr/17
$$\mathrm{5}!!=\mathrm{5}.\mathrm{3}.\mathrm{1}=\mathrm{15} \\ $$$$\mathrm{Double}\:\mathrm{factorial}\:\mathrm{for}\:\mathrm{odd}\:\mathrm{number}\:{n} \\ $$$${n}!!={n}\left({n}−\mathrm{2}\right)\left({n}−\mathrm{4}\right)…\mathrm{1} \\ $$$$\mathrm{Double}\:\mathrm{factorial}\:\mathrm{for}\:\mathrm{even}\:\mathrm{number}\:{n} \\ $$$${n}!!={n}\left({n}−\mathrm{2}\right)\left({n}−\mathrm{4}\right)…\mathrm{2} \\ $$
Commented by tawa last updated on 16/Apr/17
$$\mathrm{Now},\:\mathrm{i}\:\mathrm{understand}\:\mathrm{sir}.\:\mathrm{God}\:\mathrm{bless}\:\mathrm{you}\:\mathrm{sir}. \\ $$
Answered by Joel576 last updated on 16/Apr/17
$$\mathrm{I}\:\mathrm{think}\:\mathrm{5}!!\:=\:\mathrm{120}! \\ $$
Commented by prakash jain last updated on 16/Apr/17
$$\mathrm{5}!!\:\mathrm{is}\:\mathrm{double}\:\mathrm{factorial}\:\mathrm{of}\:\mathrm{5}=\mathrm{15}. \\ $$$$\mathrm{see}\:\mathrm{comment} \\ $$
Commented by Joel576 last updated on 16/Apr/17
$$\mathrm{thank}\:\mathrm{you} \\ $$
Commented by FilupS last updated on 17/Apr/17
$$\left(\mathrm{5}!\right)!=\mathrm{120}! \\ $$