You can study the convergence of the series sum_{n=1}^{infty} (-1)^{n}left(frac{8n}{8n+1}
ight)^{n^{2}} by using the Root Test.
First, you calculate lim_{n oinfty} sqrt[n]{|a_{n}|} to finally conclude that the series is convergent or divergent by using the Root Test.
First, you calculate lim_{n oinfty} a_{n} to finally conclude that the series is convergent or divergent.