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答: cos(2π/3) =cos(π-π/3) =-cosπ/3 =-1/2
sin 3x = 3 sin x - 4(sin x)^3 cos 3x = - 3 cos x + 4(cos x)^3 所以:(cos x)^3 =(cos 3x + 3 cos x)/ 4
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