This belongs in precalculus or prealgebra, not here. Anyway, the following are some basic definitions for you to solve your problem. If you get stuck somewhere write back, show your work and ask:

== A sequence is a geometric sequence if for any subindex k we have that is the same constant number, which is then called the quotient or ratio of the geo. seq.

== A sequence is an arithmetic sequence if for any k we have that is the same constant number d, which is then called the difference of the arit. seq.

== The sum of the first n elements of a geo. seq. with ratio q is given by

= The sum of the first n elements of an arit. seq. with difference d is given by

Tonio