$\displaystyle \sum_{k=1}^\infty \dfrac{k^{90}}{e^k}$

root test ...

$\displaystyle \lim_{k \to \infty} \left(\dfrac{k^{90}}{e^k}\right)^{1/k}$

$\displaystyle \lim_{k \to \infty} \dfrac{(k^{1/k})^{90}}{e} = \dfrac{1}{e} < 1$

therefore, converges.

I believe that the ratio test may also be used to show convergence.