How to tell what is a geometric sequence?

       Geometric Sequence is a sequence in which each term is founded by multiplying the previous term by by a constant.
       If a1 is a first term in a sequence and a2 is the second term, than: a2 = a1 * r. Also: a3 = a2 * r = a1*r². Therefore An = a1* r^(n-1). For example: sequence: 3, 9, 27, 81 ,... is a Geometric Sequence, where a1 = 3 and r = 3.