Question Number 42266 by maxmathsup by imad last updated on 21/Aug/18
$${find}\:{the}\:{value}\:{of}\:\:\sum_{{n}\:=\mathrm{0}} ^{\infty} \:\:\frac{\mathrm{1}}{{n}^{\mathrm{2}} \:+\mathrm{1}}\:\:{and}\:\sum_{{n}=\mathrm{0}} ^{\infty} \:\:\frac{\left(−\mathrm{1}\right)^{{n}} }{{n}^{\mathrm{2}} \:+\mathrm{1}}\:. \\ $$
Commented by maxmathsup by imad last updated on 22/Aug/18
$${we}\:{have}\:{proved}\:{that}\: \\ $$$${e}^{−\mid{x}\mid} \:=\frac{\mathrm{1}−{e}^{−\pi} }{\pi}\:\:+\frac{\mathrm{2}}{\pi}\:\sum_{{n}=\mathrm{1}} ^{\infty} \:\frac{\mathrm{1}−\left(−\mathrm{1}\right)^{{n}} \:{e}^{−\pi} }{{n}^{\mathrm{2}} \:+\mathrm{1}}\:{cos}\left({nx}\right) \\ $$$${x}=\mathrm{0}\:\Rightarrow\mathrm{1}\:=\:\frac{\mathrm{1}−{e}^{−\pi} }{\pi}\:+\frac{\mathrm{2}}{\pi}\:\sum_{{n}=\mathrm{1}} ^{\infty} \:\frac{\mathrm{1}}{{n}^{\mathrm{2}} \:+\mathrm{1}}\:−\frac{\mathrm{2}{e}^{−\pi} }{\pi}\:\sum_{{n}=\mathrm{1}} ^{\infty} \:\:\frac{\left(−\mathrm{1}\right)^{{n}} }{{n}^{\mathrm{2}} +\mathrm{1}}\:\:{let}\: \\ $$$${s}\:=\sum_{{n}=\mathrm{1}} ^{\infty} \:\:\frac{\mathrm{1}}{{n}^{\mathrm{2}} \:+\mathrm{1}}\:{and}\:{w}\:=\sum_{{n}=\mathrm{1}} ^{\infty} \:\:\frac{\left(−\mathrm{1}\right)^{{n}} }{{n}^{\mathrm{2}} \:+\mathrm{1}}\:\Rightarrow \\ $$$$\mathrm{1}−\frac{\mathrm{1}−{e}^{−\pi} }{\pi}\:=\frac{\mathrm{2}}{\pi}\:{s}\:−\frac{\mathrm{2}}{\pi}{e}^{−\pi} \:{w}\:\Rightarrow\frac{\pi}{\mathrm{2}}\left(\frac{\pi−\mathrm{1}+{e}^{−\pi} }{\pi}\right)={s}−{e}^{−\pi} \:{w}\:\Rightarrow \\ $$$${s}\:−{e}^{−\pi} {w}\:=\frac{\pi−\mathrm{1}+{e}^{−\pi} }{\mathrm{2}} \\ $$$${x}\:=\pi\:\Rightarrow{e}^{−\pi} \:=\frac{\mathrm{1}−{e}^{−\pi} }{\pi}\:+\frac{\mathrm{2}}{\pi}\:\sum_{{n}=\mathrm{1}} ^{\infty} \:\frac{\left(−\mathrm{1}\right)^{{n}} }{{n}^{\mathrm{2}} \:+\mathrm{1}}\:−\frac{\mathrm{2}{e}^{−\pi} }{\pi}\:\sum_{{n}=\mathrm{1}} ^{\infty} \:\:\frac{\mathrm{1}}{{n}^{\mathrm{2}} \:+\mathrm{1}}\:\Rightarrow \\ $$$${e}^{−\pi} \:−\frac{\mathrm{1}−{e}^{−\pi} }{\pi}\:=\frac{\mathrm{2}}{\pi}{w}\:−\frac{\mathrm{2}{e}^{−\pi} }{\pi}\:{s}\:\Rightarrow\frac{\pi}{\mathrm{2}}\left(\frac{\pi\:{e}^{−\pi} \:−\mathrm{1}\:+{e}^{−\pi} }{\pi}\right)\:=−\mathrm{2}{e}^{−\pi} \:{s}\:+{w}\:\Rightarrow \\ $$$$\frac{\pi\:{e}^{−\pi} \:−\mathrm{1}+{e}^{−\pi} }{\mathrm{2}}\:\:=−\mathrm{2}\:{e}^{−\pi} {s}\:+{w}\:\:{we}\:{get}\:{the}\:{system} \\ $$$$\boldsymbol{\mathrm{s}}\:−\boldsymbol{\mathrm{e}}^{−\pi} {w}\:=\frac{\pi−\mathrm{1}\:+{e}^{−\pi} }{\mathrm{2}}\:\:{and}\:\:−\mathrm{2}\:{e}^{−\pi} \:{s}\:+{w}\:=\frac{\left(\pi+\mathrm{1}\right){e}^{−\pi} }{\mathrm{2}}\:\Rightarrow \\ $$$$−\mathrm{2}\:{e}^{−\mathrm{2}\pi} \:{s}\:+{e}^{−\pi} {w}\:=\frac{\left(\pi+\mathrm{1}\right){e}^{−\mathrm{2}\pi} }{\mathrm{2}}\:\Rightarrow \\ $$$$\left(\mathrm{1}−\mathrm{2}{e}^{−\mathrm{2}\pi} \right){s}\:=\:\frac{\pi−\mathrm{1}+{e}^{−\pi} \:\:+\left(\pi+\mathrm{1}\right){e}^{−\mathrm{2}\pi} }{\mathrm{2}}\:\Rightarrow \\ $$$${s}\:=\frac{\pi−\mathrm{1}\:+{e}^{−\pi} \:+\left(\pi+\mathrm{1}\right){e}^{−\mathrm{2}\pi} }{\mathrm{2}\left(\mathrm{1}−\mathrm{2}\:{e}^{−\mathrm{2}\pi} \right)}\:\:\:\:{and}\:\:{w}\:=\frac{\left(\pi+\mathrm{1}\right){e}^{−\pi} }{\mathrm{2}}\:+\mathrm{2}\:{e}^{−\pi} \:{s}\:. \\ $$