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9. Sequence: 2, 4, 6, 8, 10, ...
- The sequence is an arithmetic sequence with the first term \( a_1 = 2 \) and a common difference \( d = 2 \).
- \( a_n = 2n \).

10. Sequence: 1, -1, 1, -1, 1, ...
- The sequence alternates between 1 and -1.
- \( a_n = (-1)^{n+1} \).

11. Sequence: 3, 9, 27, 81, ...
- The sequence is a geometric sequence with the first term \( a_1 = 3 \) and a common ratio \( r = 3 \).
- \( a_n = 3^n \).

12. Sequence: \( \frac{1}{2 \times 1}, \frac{1}{3 \times 2}, \frac{1}{4 \times 3}, \frac{1}{5 \times 4}, \dots \)
- The sequence can be represented as \( a_n = \frac{1}{(n+1) \times n} \).