ĐKXĐ: \(sin4x\ne0\Rightarrow...\)
\(\frac{1}{cosx}=\frac{2}{2sin2x.cos2x}-\frac{1}{sin2x}\)
\(\Leftrightarrow\frac{1}{cosx}=\frac{1}{sin2x}\left(\frac{1}{cos2x}-1\right)\)
\(\Leftrightarrow\frac{1}{cosx}=\frac{1}{sin2x}\left(\frac{2sin^2x}{cos2x}\right)=\frac{2sin^2x}{2sinx.cosx.cos2x}\)
\(\Leftrightarrow1=\frac{sinx}{cos2x}\)
\(\Leftrightarrow sinx=cos2x=sin\left(\frac{\pi}{2}-2x\right)\)
\(\Leftrightarrow...\)