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The sum of the first n terms of a GP (a, ar, ar², …) when r ≠ 1 is
AS_n = a · (1 − rⁿ) / (1 − r)
BS_n = a + (n − 1)r
CS_n = a · r^(n−1)
DS_n = n(n + 1)/2
Answer & Solution
Correct answer: A. S_n = a · (1 − rⁿ) / (1 − r)
S_n = a(1 − rⁿ)/(1 − r), equivalent to a(rⁿ − 1)/(r − 1) for |r| ≠ 1.
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