Question Number 24540 by Tinkutara last updated on 20/Nov/17
$${Prove}\:{that} \\ $$$$\left({i}\right)\:\underset{{n}=\mathrm{0}} {\overset{\infty} {\sum}}\frac{{n}^{\mathrm{2}} }{{n}!}=\mathrm{2}{e}. \\ $$$$\left({ii}\right)\:\underset{{n}=\mathrm{0}} {\overset{\infty} {\sum}}\frac{{n}^{\mathrm{3}} }{{n}!}=\mathrm{5}{e}. \\ $$$$\left({iii}\right)\:\underset{{n}=\mathrm{0}} {\overset{\infty} {\sum}}\frac{{n}^{\mathrm{4}} }{{n}!}=\mathrm{15}{e}. \\ $$
Commented by prakash jain last updated on 20/Nov/17
$$\mathrm{Write}\:\mathrm{a}\:\mathrm{polynomial}\:\mathrm{in}\:{n}\:{P}\left({n}\right)\:{of} \\ $$$${degree}\:{k}\:{as} \\ $$$${P}\left({n}\right)={a}_{\mathrm{0}} +{a}_{\mathrm{1}} {n}+{a}_{\mathrm{2}} {n}\left({n}−\mathrm{1}\right)+..+{a}_{{k}} {n}\left({n}−\mathrm{1}\right)…\left({n}−{k}+\mathrm{1}\right) \\ $$$${solve}\:{for}\:{a}_{\mathrm{0}} ,..{a}_{{k}} \\ $$$${and}\:{continue}\:{as}\:{in}\:{case}\:{of}\:{n}^{\mathrm{2}} \:{in}\:{answer} \\ $$
Answered by prakash jain last updated on 20/Nov/17
$${n}^{\mathrm{2}} ={n}+{n}\left({n}−\mathrm{1}\right) \\ $$$$\underset{{n}=\mathrm{0}} {\overset{\infty} {\sum}}\frac{{n}+{n}\left({n}−\mathrm{1}\right)}{{n}!} \\ $$$$=\underset{{n}=\mathrm{1}} {\overset{\infty} {\sum}}\frac{\mathrm{1}}{\left({n}−\mathrm{1}\right)!}+\underset{{n}=\mathrm{2}} {\overset{\infty} {\sum}}\frac{\mathrm{1}}{\left({n}−\mathrm{2}\right)!} \\ $$$$={e}+{e}=\mathrm{2}{e} \\ $$
Commented by Tinkutara last updated on 21/Nov/17
$$\mathrm{Thank}\:\mathrm{you}\:\mathrm{very}\:\mathrm{much}\:\mathrm{Sir}! \\ $$
Answered by ajfour last updated on 20/Nov/17
$$\left(\boldsymbol{{i}}\right)\:\:\:\:\:{xe}^{{x}} ={x}+\frac{{x}^{\mathrm{2}} }{\mathrm{1}!}+\frac{{x}^{\mathrm{3}} }{\mathrm{2}!}+\frac{{x}^{\mathrm{4}} }{\mathrm{3}!}+…. \\ $$$$\:\:\:\:\:\frac{{d}}{{dx}}\left({xe}^{{x}} \right)\:=\:\mathrm{1}\:+\frac{\mathrm{2}{x}}{\mathrm{1}!}\:\:+\:\frac{\mathrm{3}{x}^{\mathrm{2}} }{\mathrm{2}!}+\:\frac{\mathrm{4}{x}^{\mathrm{3}} }{\mathrm{3}!}+… \\ $$$$\Rightarrow\:{e}^{{x}} +{xe}^{{x}} \:=\frac{\mathrm{1}^{\mathrm{2}} }{\mathrm{1}!}+\frac{\mathrm{2}^{\mathrm{2}} {x}}{\mathrm{2}!}+\frac{\mathrm{3}^{\mathrm{2}} {x}^{\mathrm{2}} }{\mathrm{3}!}+\frac{\mathrm{4}^{\mathrm{2}} {x}^{\mathrm{3}} }{\mathrm{4}!}+.. \\ $$$${With}\:\boldsymbol{{x}}=\mathrm{1}\:\:{we}\:{get} \\ $$$$\:\:\:\:\mathrm{2}\boldsymbol{{e}}\:=\:\underset{{n}=\mathrm{0}} {\overset{\infty} {\sum}}\:\frac{{n}^{\mathrm{2}} }{{n}!}\:. \\ $$$$ \\ $$
Commented by Tinkutara last updated on 21/Nov/17
$$\mathrm{Thank}\:\mathrm{you}\:\mathrm{very}\:\mathrm{much}\:\mathrm{Sir}! \\ $$