Arithmetic sequence: a1 + A5 + A9 = 2 π, is the value of COS (A2 + A8) -?

Because the arithmetic sequence: a1 + A5 + A9 = 2 π, that is, 3A5 = 2 π, then A5 = 2 π / 3 = (A2 + A8) / 2, so A2 + A8 = 4 π / 3, so cos (A2 + A8) = Cos4 π / 3 = - 1 / 2

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