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A common ratio is a term which applies to a geometric sequence. A geometric sequence starts from any number `a` and any next term is equal to the previous term multiplied by a fixed number `r.` This number `r` is called the *common ratio* of a geometric sequence.

Consider the given sequence. It starts from the number 4. The next number is 12/4=3 times greater than the first. So if this sequence is a geometric one, then the next term must be 12*3=36. This is true, and the next term must be 36*3=108, which is also true.

Therefore this sequence is a geometric one and its **common ratio is 3**. We can predict its next terms: 108*3=324, 324*3=972 and so on. The n-th term is

`a_n=4*3^(n-1).`

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