If the general term of sequence {a n} a n = n & # 178; (COS & # 178; n π / 3-sin & # 178; n π / 3), the sum of the first n terms is Sn, then S30 is?

If the general term of sequence {a n} a n = n & # 178; (COS & # 178; n π / 3-sin & # 178; n π / 3), the sum of the first n terms is Sn, then S30 is?

An = n & # 178; (COS & # 178; n π / 3-sin & # 178; n π / 3) = n & # 178; * cos (2n π / 3) for the function f (n) = cos (2n π / 3), period T = 2 π / (2 π / 3) = 3, we can see that when n = 3K + 1, K belongs to N, an = (3K + 1) & # 178; * cos [2 (3K + 1) π / 3] = (3K + 1) & # 178; * cos (2k π + 2 π / 3) = (3K + 1) & # 178

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