If sin x + sin2x + sin3x= 1, then cos6x-4cos4x+8cos2x =
need a proper solution and very fast. got coaching test tomorrow.

Dear student,

You can just solve that cubic numerically and get sin(x) = 0.5437 . Then using cos^2(x) = 1- sin^2(x) you can easily evaluate it. The answer is  4.

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