Ex 9.4, 6 - Find sum 3 x 8 + 6 x 11 + 9 x 14 - Chapter 9 Class 11 - Finding sum from nth number

Ex 9.4, 6 - Chapter 9 Class 11 Sequences and Series - Part 2
Ex 9.4, 6 - Chapter 9 Class 11 Sequences and Series - Part 3

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Transcript

Question 6 Find the sum to n terms of the series 3 8 + 6 11 + 9 14 + Step 1: Finding nth term an an = (nth term of 3, 6, 9 ) (nth term of 8, 11, 14, ) an = (3n)(3n + 5) = 9n2 + 15n Step 2: Finding sum of n terms = 9 (n(n+1)(2n+1))/6 +15 (n(n+1))/2 = 9 ( + 1)(2 + 1)/6 + (15 ( + 1))/2 = (3n(n+1)(2n+1))/2 + (15n(n+1))/2 = (3n(n+1))/2 [(2n+1)+5] = (3n(n+1))/2 [2n+1+5] = (3n(n+1))/2 [2n+6] = 3/2n(n+1) 2(n+3) = 3n(n + 1)(n + 3) Hence, The required sum is 3n(n + 1)(n + 3)

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Davneet Singh has done his B.Tech from Indian Institute of Technology, Kanpur. He has been teaching from the past 13 years. He provides courses for Maths, Science, Social Science, Physics, Chemistry, Computer Science at Teachoo.