Which of the following sequences is an arithmetic sequence? Why?
1. 3, 7, 11, 15, 19
2. 4, 16, 64, 256
3. 48, 24, 12, 6, 3, ...
4. 1, 4, 9, 16, 25, 36
5. 1, 1/2, 0, -1/2
6. -2, 4, -8, 16, ...
7. 1, 0, -1, -2, -3
8. 1/2, 1/3, 1/4, 1/5, ...
9. 3x, x, x/3, x/9, ...
10. 9.5, 7.5, 5.5, 3.5, ..