## Calculus 8th Edition

$\Sigma_{n=1}^{\infty}a_{n}=\Sigma_{n=1}^{\infty}\frac{(-1)^{n}}{coshn}$ Note that $a_{n}=\frac{1}{coshn}$ is monotonically decreasing. It is because $cohn$ is monotonically increasing for $n\gt 0$ The given series is convergent.