Question

Is 3, 6, 9, 12 ,15, 18 a geometric sequence?

Answer 1

No, it's an arithmetic sequence with initial term `3` and common difference `3`.

Explanation:

A geometric sequence has a common ratio between terms.

An arithmetic sequence has a common difference between terms.

If a sequence of Real numbers is both a geometric sequence and an arithmetic sequence then it is constant.

Note I say Real numbers. The sequence `1, -1,1,-1,...` is both an arithmetic and geometric sequence in modulo `4` arithmetic, with common difference `2` and common ratio `-1`.