If the sum of the first n terms of the sequence an is Sn, then S30 =?

An = n & sup2; (COS & sup2; n π / 3-sin & sup2; n π / 3) = n ^ 2 * cos (2n π / 3) (double angle formula) cos (2 π / 3) = - 1 / 2cos (4 π / 3) = - 1 / 2cos (6 π / 3) = 1, so a (3K-2) + a (3K-1) + a (3K) = (3K-2) ^ 2 * (- 1 / 2) + (3K-1) ^ 2 * (- 1 / 2) + (3K) ^ 2 * 1 = 9k-5 / 2, so S30 = a1 +

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