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Class 9 Mathematics Sequences and Progressions Sum of first n natural numbersHard

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If (S_n-S_{n-5}=390), what will be the value of (n)?

Answer and explanation

Correct answer: (78)

The difference is ((n-4)+(n-3)+(n-2)+(n-1)+n=5n-10), so (5n-10=390) gives (n=80). Form the sum of the last five terms.

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Math Sequences Natural-Numbers Sum Sum Of First N Natural Numbers Sequences And Progressions Mathematics Class 9 Mcq

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