4. Find the sum of the following series. \text { (a) } \sum_{n=1}^{\infty} \frac{1}{e^{n}} \text { (b) } \sum_{n=1}^{\infty} \frac{(-1)^{n}+3 \cdot 5^{n}}{6^{n}} \text { (c) } \sum_{n=1}^{\infty} \frac{1}{n^{2}+n} \text {

(d) } \sum_{n=2}^{\infty} \frac{1}{n^{2}+3 n+2}