Cos^2 (2x) + cos^2 (6x) = 1

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Помогите))

Cos^2 (2x) = (1+cos (4x)) / 2

Cos^2 (6x) = (1+cos (12x)) / 2

Cos^2 (2x) + cos^2 (6x) = 1

(1+cos (4x)) / 2 + (1+cos (12x)) / 2=1

cos (4x) / 2+cos (12x) / 2=0

cos (4x) + cos (12x) = 0

2*cos ((4x+12 х) / 2) * cos ((4x-12 х) / 2) = 0

cos (8 х) * cos (4x) = 0

8 х=pi*k или 4 х=pi*n

ответ х=pi*k/8