2.10.99 Number series

Condition: Examine series for convergence: a) \( \sum_{n=1}^{\infty} \frac{4^{n}+5^{n}}{20^{n}} \), b) \( \sum_{n=1}^{\infty} \frac{n^{2}}{n^{7}+7} \), c) \( \sum_{n=1}^{\infty} \tan \frac{\pi}{3^{n}} \).