Which of the following is an alternating series?
∑n=1∞1n2\sum_{n=1}^{\infty} \frac{1}{n^2}∑n=1∞n21
∑n=1∞(−1)nn\sum_{n=1}^{\infty} \frac{(-1)^n}{n}∑n=1∞n(−1)n
∑n=1∞n2n\sum_{n=1}^{\infty} \frac{n}{2^n}∑n=1∞2nn
∑n=1∞cos(n)\sum_{n=1}^{\infty} \cos(n)∑n=1∞cos(n)