Just as arithmetic sequences have a
common difference, geometric sequences
have a common ratio which is denoted with
the letter r. The common ratio is found by
dividing successive terms in the sequence.
So in our geometric sequence example, we
could use 9/3 = 3 or 243/81=3 to find that r
= 3. As with finding a common difference,
when we find a common ratio, we must use
the term that appears later in the sequence
as our numerator and the number right
before it as our denominator[2].