Which rule defines Sn for an = 3(0.7)n ?

Since aₙ = 3(0.7)ⁿ, the first term is a₁ = 2.1 and the common ratio is 0.7. The sum of the first n terms is Sₙ = a₁(1 − rⁿ)/(1 − r) = 2.1(1 − 0.7ⁿ)/0.3 = 7(1 − 0.7ⁿ). This is the rule in choice C.

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