If the sum of the first n terms of an AP is

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Solution

Q.
If the sum of the first n terms of an AP is 3n² + n and its common difference is 6, then its first term is:

1 2

2 3

3 1

4 4

Ans.
(4)

$W e h a v e, S ₙ = 3 n ² + n \Rightarrow n / 2 (2 a + (n " - " 1) \times 6) = 3 n^{2} + n \Rightarrow a + 3 n - 3 = 3 n + 1 \Rightarrow a = 1 + 3 \Rightarrow a = 4$

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