bạn tham khảo nha
\(\Leftrightarrow\dfrac{1-cos2x}{2}+\dfrac{1-cos6x}{2}-1-cos4x=0\\ \Leftrightarrow1-cos2x+1-cos6x-2-2cos4x=0\\ \Leftrightarrow cos2x+cos6x+2cos4x=0\\ \Leftrightarrow cos4x.cos2x+cos4x=0\\ \Leftrightarrow cos4x\left(cos2x+1\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x=\dfrac{\pi}{8}+\dfrac{k\pi}{4}\\x=\dfrac{\pi}{2}+k\pi\end{matrix}\right.\)
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