Question Number 28826 by abdo imad last updated on 30/Jan/18
$${let}\:{give}\:{f}\left({x}\right)=\:{e}^{−{x}} \:{cosx}\:\:{and}\:\mathrm{2}\pi\:{periodic} \\ $$$$\left.\mathrm{1}\right)\:{developp}\:{f}\:{at}\:{fourier}\:{series} \\ $$$$\left.\mathrm{2}\right)\:{find}\:{the}\:{value}\:{of}\:\:\sum_{{n}=−\infty} ^{{n}=+\infty} \:\:\:\frac{\left(−\mathrm{1}\right)^{{n}} }{\mathrm{1}+{n}^{\mathrm{2}} }\:. \\ $$