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Class 9MathematicsSequences and ProgressionsSum of first n natural numbersMediumLevel 60
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For which (n) will (S_n=780)?

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Answer and explanation

Correct answer: (39)

(S_{39}=\frac{39\times40}{2}=780), so (n=39). In such questions, match (n(n+1)) with (2S_n).

Related tags

MathSequencesNatural-NumbersSum

Frequently asked questions

What is the correct answer to this question?

(39)

Why is this the correct answer?

(S_{39}=\frac{39\times40}{2}=780), so (n=39). In such questions, match (n(n+1)) with (2S_n).

Which subject and chapter does this question cover?

This is a Class 9 Mathematics question. Chapter: Sequences and Progressions. Topic: Sum of first n natural numbers.

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  2. 2What will be the 9th term in the sequence (3, −6, 12, −24, …)?
  3. 3If (a=10) and (d=2), what will be the sum of the first (6) terms?
  4. 4If a = 7 and d = 3, what is the sum of the first 6 terms?
  5. 5What is the sum of the first 6 terms of the arithmetic progression (15, 21, 27, 33, ...)?
  6. 6What is the sum of the first (10) natural numbers?
  7. 7Find the sum of the first (20) natural numbers.
  8. 8If \(S_n=\frac{n(n+1)}{2}\), what is \(S_{15}\)?
  9. 9What is the value of (1+2+3+\cdots+25)?
  10. 10Which is the sum of the first (30) natural numbers?
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