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If (n=50), what is the value of (S_{2n}-S_n)?

Answer and explanation

Correct answer: 3775

Substituting n=50 gives 2n=100. The sum of the first n natural numbers is \(S_n=\frac{n(n+1)}{2}\). Thus, \(S_{100}=\frac{100\times101}{2}=5050\) and \(S_{50}=\frac{50\times51}{2}=1275\). Therefore, \(S_{2n}-S_n=5050-1275=3775\). An option such as 3875 results from an error in subtraction or in applying the sum formula. Exam tip: evaluate the index 2n first, then find the two sums separately.

Related tags

Sequences And ProgressionsSum Of Natural NumbersArithmetic SeriesSubstitutionIndex NotationSum Of First N Natural NumbersMathematicsClass 9 Mcq

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