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demon311

$\cos 6x +\cos 4x +\cos 2x +1=0 \\
\Leftrightarrow 4\cos^3 2x +2\cos^2 2x - 2\cos 2x=0 \\
\Leftrightarrow \cos 2x (\cos 2x+1)(4\cos 2x-2)=0$
 
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