- #1

karush

Gold Member

MHB

- 3,269

- 5

$$\sum_{n=1}^{\infty}\dfrac{(-2)^n}{n^2}$$

If $\displaystyle\lim_{n \to \infty}

\left|\dfrac{a_{n+1}}{a_n}\right|=L>1

\textit{ or }

\left|\dfrac{a_{n+1}}{a_n}\right|=\infty

\textit{ then the series } \sum_{n=1}^{\infty}a_n \textit{ is divergent}$

ok I spent about half hour trying to do this but not sure what a is

also this one if can...

$$\sum_{n=1}^{\infty} \frac{(2n)!}{9^n n!}$$