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NCERT Class XI Mathematics - Sequences and Series - Solutions

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Question : 77
Total: 106
n2+2n
Solution:  
We have, an = n2+2n
Hence, the sum to n terms is,
Sn =
n
Σ
k=1
ak =
n
Σ
k=1
(k2+2k) =
n
Σ
k=1
k2+
n
Σ
k=1
2k
=
n(n+1)(2n+1)
6
 + (2+22+23+...+2n) =
n(n+1)(2n+1)
6
 + 2(2n−1)
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