Solving Sequences and Series Homework

[Char. Limit]

Homework Statement

So I was helping my roommate with his homework, and it has the following problem:

Fill in the blanks so that

___, 8, ___, ___, 27, ___, ...

is

a. an arithmetic sequence.
b. a geometric sequence.
c. a sequence that is neither arithmetic nor geometric, for which you are able to write a general term.

Homework Equations

The Attempt at a Solution

We tried a Fibonnaci-type sequence, but that really didn't work. And we don't know any other types of sequences. Should I try some sort of quadratic relationship?

 

[Char. Limit]

Never mind, solved.