Ответы на вопрос:
* * * cos(α + β) = cosαcosβ - sinαsinβ ;
cos(α - β) = cosαcosβ + sinαsinβ ; cos(2π/3) = - 1/2 * * *
cosα + cos(2π/3 + α) + cos(2π/3 - α) =
= cosα + cos(2π/3)cosα - sin(2π/3)sinα + cos(2π/3)cosα + sin(2π/3)sinα =
= cos + cos(2π/3)cosα + cos(2π/3)cosα = cosα - 1/2 · cosα - 1/2 · cosα =
= 1/2 · cosα - 1/2 · cosα = 0
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