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The sum of a geometric sequence, that is, a series a1 + a2 + a3 + ... in which the terms form a geometric sequence.
The sum of the first n terms can be obtained as follows:
Sn = a1 + a2 + a3 + ...an = a1 + a1r + a1r2 + ... + a1rn-1 = a1 (rn - 1)/(r - 1) in which r is the common ratio.
