If the sum of the first n terms of the sequence an is Sn, then Sn =?

If the sum of the first n terms of the sequence an is Sn, then Sn =?

∵ the general term a [n] = n ^ 2 [(Cosn π / 3) ^ 2 - (Sinn π / 3) ^ 2], the sum of the first n terms is s [n] ∵ a [n] = n ^ 2cos (2n π / 3) ∵ s [n] = 1 ^ 2 (- 1 / 2) + 2 ^ 2 (- 1 / 2) + 3 ^ 2 + 4 ^ 2 (- 1 / 2) + 5 ^ 2 (- 1 / 2) + 6 ^ 2 +... + n ^ 2cos (2n π / 3) when n = 3K-2, that is: k = (n + 2) / 3: s [3K-2] = (- 1 / 2) = (- 1 / 2)

Elden Ring Premiere

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