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If Sₙ + Sₙ₋₁ = 12544 and Sₙ − Sₙ₋₁ = 112, what is the value of Sₙ?

Answer and explanation

Correct answer: 6328

Treat the two statements as simultaneous equations. Add them to eliminate Sₙ₋₁: (Sₙ + Sₙ₋₁) + (Sₙ − Sₙ₋₁) = 12544 + 112. The Sₙ₋₁ terms cancel, leaving 2Sₙ = 12656. Dividing by 2 gives Sₙ = 6328, so option C is correct. The result can be checked by finding the other partial sum: Sₙ₋₁ = 12544 − 6328 = 6216. Then 6328 + 6216 = 12544 and 6328 − 6216 = 112, so both original conditions hold. Option A is the value of Sₙ₋₁, a likely distractor. Options B and D arise from an arithmetic or division error and fail the verification.

Related tags

Sum Of Natural NumbersPartial SumsSimultaneous EquationsSequencesSum Of First N Natural NumbersSequences And ProgressionsMathematicsClass 9 Mcq

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