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If Sₙ + Sₙ₋₁ = 5625 and Sₙ − Sₙ₋₁ = 75, what is the value of Sₙ?

Answer and explanation

Correct answer: 2850

This is a simultaneous-equation problem involving two consecutive partial sums. Add the equations so that Sₙ₋₁ is eliminated: (Sₙ + Sₙ₋₁) + (Sₙ − Sₙ₋₁) = 5625 + 75. The middle terms cancel, giving 2Sₙ = 5700. Dividing both sides by 2 gives Sₙ = 2850, so option B is correct. Substitution verifies the result: if Sₙ = 2850, then Sₙ₋₁ = 5625 − 2850 = 2775, and 2850 − 2775 = 75. Option A is the corresponding previous partial sum, which is a plausible result of stopping after elimination. Options C and D do not satisfy both given equations, so they are not valid values of Sₙ.

Related tags

Sum Of Natural NumbersSequencesSimultaneous EquationsPartial SumsSum Of First N Natural NumbersSequences And ProgressionsMathematicsClass 9 Mcq

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