Use the formula Sn =a (1 - rn ) ÷ (1 – r) to find the sum Sn for each of the infinite and finite geometric series.

9. The first 9 terms of 12, – 6 ,+ 3, - …

10. The first 15 terms of -1, + 1, -…

11. The first 6 terms of 1, + 3, + …

12. The first 10 terms of 2, – 4, + …