Which of these series is a geometric series?
∑n2n\sum \frac{n}{2^n}∑2nn
∑(13)n\sum (\frac{1}{3})^n∑(31)n
∑1n2\sum \frac{1}{n^2}∑n21
∑n!2n\sum \frac{n!}{2^n}∑2nn!