ĐKXĐ: \(cos2x\ne0\)
\(\dfrac{cos4x}{cos2x}=\dfrac{sin2x}{cos2x}\)
\(\Rightarrow cos4x=sin2x\)
\(\Leftrightarrow cos4x=cos\left(\dfrac{\pi}{2}-2x\right)\)
\(\Leftrightarrow\left[{}\begin{matrix}4x=\dfrac{\pi}{2}-2x+k2\pi\\4x=2x-\dfrac{\pi}{2}+k2\pi\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{\pi}{12}+\dfrac{k\pi}{3}\\x=-\dfrac{\pi}{4}+k\pi\end{matrix}\right.\) \(\Rightarrow x=\left\{\dfrac{\pi}{12};\dfrac{5\pi}{12}\right\}\) có 2 nghiệm
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