Ex 9.4, 9 - Find sum of series, nth term is n2 + 2n - Class 11 - Finding sum from nth number

  1. Chapter 9 Class 11 Sequences and Series
  2. Serial order wise
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Ex 9.4, 9 Find the sum to n terms of the series whose nth terms is given by n2 + 2n Given an = n2 + 2n Now, sum of n terms is Now, = 2 + 4 + 8 + … + 2n This is GP with first term A = 2 & common ratio R = 4/2 = 2 We know that sum of n term in GP Sn = (𝐴(R^n−1))/(R−1) Putting values Now from (1) Putting values = (n(n+1)(2n+1))/6 + 2(2n – 1) Thus, the required sum is (n(n+1)(2n+1))/6 + 2(2n – 1)

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Davneet Singh
Davneet Singh is a graduate from Indian Institute of Technology, Kanpur. He provides courses for Mathematics from Class 9 to 12. You can ask questions here.
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