1. 25, 5 , 1 geometric..... an = 25(1/5)(n - 1)
2. 25, 19, 13.....arithmetic.....an = 25 - 6(n - 1) = 31 - 6n
3. 4, 9 , 16 .......neither.. we have no common difference or common ratio...but if we changed it to 4, 10, 16.....then arithmetic ....an = 4 + 6(n - 1) = -2 + 6n
4. 50, 60, 70 ......arithmetic.....an = 50 + 10(n - 1) = 40 + 10n
5. 1/2, 3, 18 .....geometric.......an = (1/2)(6)^(n - 1)