Express Cosα +cosβ+cosγ+cos(α+β+γ) as the product of three cosines

Express Cosα +cosβ+cosγ+cos(α+β+γ) as the product of three cosines Express Cosα +cosβ+cosγ+cos(α+β+γ) as the product of three cosines $Cos\alpha + cos\beta + …

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