Question Number 28618 by abdo imad last updated on 27/Jan/18
$${let}\:{give}\:{u}_{{n}} =\:\sum_{{k}={n}} ^{+\infty} \:\:\frac{\left(−\mathrm{1}\right)^{{k}} }{\sqrt{{k}+\mathrm{1}}}\:\:{study}\:{the}\:{convergence}\:{of}\: \\ $$$$\Sigma\:{u}_{{n}} . \\ $$