In mathematics, a geometric progression, is a sequence of numbers where each term after the first is found by multiplying the previous one by a fixed number(The fixed number is non-zero number), it is also known as a geometric sequence.
The sequence 1, 2, 4, 8, 16, ... is a geometric progression, the common ratio is 2. the common ration may be negative, the sequence will switchs from positive to negative and back, like the sequence 1, -2, 4, -8, ..., the common ration is -2.
an = [a1 r (n-1)]
sn = (a1(1-rn))(1-r)
Where, a1 - first term, n - number of term, r - common difference.