The natural nos. r grouped as (13), (57911), (13,15,17,21,23),........
Find the sum of the nos. in nth group?
1) 4n3
2) 3n2
3) 2n2
4) n
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1 Answers
62The groups are each of size 2n
Your third group should be 13, 15, 17, 19, 21, 23 (correction)
After that, the number of terms upto the first n groups = 2+4+6+... 2n = n(n+1)
The number of terms upto the first n-1 groups is n(n-1)
Sum of the first n(n+1) odd numbers is.... n2(n+1)2
Sum of the first n(n-1) odd numbers is.... n2(n-1)2
Hence the sum of elements in the nth group = n2(n+1)2-n2(n-1)2 = 4n3
