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What is the value of (S_{36}+S_{72}-S_{18})?

Answer and explanation

Correct answer: 3123

Here, \(S_n\) denotes the sum of the first \(n\) natural numbers, so \(S_n=\frac{n(n+1)}{2}\). Thus, \(S_{36}=666\), \(S_{72}=2628\), and \(S_{18}=171\). Therefore, \(S_{36}+S_{72}-S_{18}=666+2628-171=3123\). The option 3143 can result from an error of 20 while subtracting. Exam tip: calculate each \(S_n\) separately before performing the final addition and subtraction.

Related tags

MathematicsSequences And ProgressionsSum Of Natural NumbersArithmetic SeriesFormula ApplicationSum Of First N Natural NumbersClass 9 Mcq

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