Question Number 49982 by pieroo last updated on 12/Dec/18
$$\mathrm{If}\:\frac{\mathrm{n}_{\mathrm{P}_{\mathrm{r}} } }{\mathrm{n}_{\mathrm{C}_{\mathrm{r}} } }\:−\mathrm{1}=\mathrm{5},\:\mathrm{find}\:\mathrm{the}\:\mathrm{value}\:\mathrm{of}\:\mathrm{r} \\ $$
Answered by ajfour last updated on 13/Dec/18
$$\:^{{n}} {P}_{{r}} \:=\:\frac{{n}!}{\left({n}−{r}\right)!}\:\:\:\&\:\:\:\:^{{n}} {C}_{{r}} \:=\:\frac{{n}!}{{r}!\left({n}−{r}\right)!} \\ $$$$\Rightarrow\:\:\:\frac{\:^{{n}} {P}_{{r}} \:}{\:^{{n}} {C}_{{r}} }−\mathrm{1}\:=\:{r}!−\mathrm{1}\:=\:\mathrm{5} \\ $$$$\Rightarrow\:\:\:{r}!\:=\:\mathrm{6}\:\:\:\Rightarrow\:\:\:{r}=\:\mathrm{3}\:. \\ $$
Commented by ajfour last updated on 13/Dec/18
$$\mathrm{2}!\:=\:\mathrm{2}\:\:,\:\:\mathrm{4}!\:=\:\mathrm{24} \\ $$$$\Rightarrow\:\:\mathrm{2}!\:<{r}!\:<\:\mathrm{4}! \\ $$$${lets}\:{check}\:\mathrm{3}!\:=\:\mathrm{1}×\mathrm{2}×\mathrm{3}\:=\:\mathrm{6} \\ $$$$\Rightarrow\:\:{r}!\:=\:\mathrm{6}\:\:\Rightarrow\:\:{r}=\mathrm{3}\:. \\ $$
Commented by pieroo last updated on 13/Dec/18
$$\mathrm{thanks}\:\mathrm{sir},\:\mathrm{But}\:\mathrm{how}\:\mathrm{do}\:\mathrm{i}\:\mathrm{explain}\:\mathrm{to}\: \\ $$$$\mathrm{my}\:\mathrm{younger}\:\mathrm{brother}\:\mathrm{that}\:\mathrm{r}!=\mathrm{6}\:\Rightarrow\mathrm{r}=\mathrm{3}? \\ $$
Commented by pieroo last updated on 13/Dec/18
$$\mathrm{wow},\:\mathrm{thank}\:\mathrm{you}\:\mathrm{very}\:\mathrm{much}\:\mathrm{for}\:\mathrm{your}\:\mathrm{patience}. \\ $$