Which of these series is a telescoping series?
∑n=1∞(1n−1n+1)\sum_{n=1}^{\infty} (\frac{1}{n} - \frac{1}{n+1})∑n=1∞(n1−n+11)
∑n=1∞(12)n\sum_{n=1}^{\infty} (\frac{1}{2})^n∑n=1∞(21)n
∑n=1∞1n2\sum_{n=1}^{\infty} \frac{1}{n^2}∑n=1∞n21
∑n=1∞(−1)n\sum_{n=1}^{\infty} (-1)^n∑n=1∞(−1)n