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Sum to n terms of the series 7 + 77 + 777 + … equals
A10ⁿ − 1
B7 · (10ⁿ − 1) / 9
C(7/9) · [10(10ⁿ − 1)/9 − n]
D7n(n + 1)/2
Answer & Solution
Correct answer: C. (7/9) · [10(10ⁿ − 1)/9 − n]
Write each term as 7/9 × (10^k − 1). Summing: S_n = (7/9)[10 + 10² + … + 10ⁿ − n] = (7/9)[10(10ⁿ − 1)/9 − n].
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