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].