Question Number 28833 by abdo imad last updated on 30/Jan/18
$${let}\:{give}\:\varphi\left({x}\right)\:={x}\:,\varphi\:\mathrm{2}\pi\:{periodique}\:{even} \\ $$$${developp}\:{f}\:{at}\:{fourier}\:{series}\:{then}\:{find}\:{the}\:{value}\:{of} \\ $$$$\sum_{{n}=\mathrm{1}} ^{\infty} \:\:\frac{\left(−\mathrm{1}\right)^{{n}} }{{n}^{\mathrm{2}} }\:{and}\:\:\sum_{{n}=\mathrm{0}} ^{\infty} \:\:\frac{\mathrm{1}}{\left(\mathrm{2}{n}+\mathrm{1}\right)^{\mathrm{2}} }\:. \\ $$