Key idea:
1) If the given sequence does have common difference, or common ratio, then it is neither an arithmatic nor geometric.
2) How to find out :
We say "d" is common difference of arithmatic :
If d value is (a2 - a1) ≠ (a3 - a2 ) then it is not arithmatic sequences.
We say "r" is common ratio of geomatric sequences:
If r value is (a1 / a 2) ≠ (a2 / a3 ) , then it is not geomatric sequences.
... Here, we get common ratio of geomatric sequence = (1/4) /(1/6) = (1/6) /(1/9) = (3/2) , Then it is geometric sequences.
3) Is (3/2) greater than 1?
4) If geometric common ratio is greater than 1 , it means the sum of the series keep increasing , then we say it is diverges.