ĐKXĐ: ...
\(\Leftrightarrow\left\{{}\begin{matrix}cosx\ge0\\cos^2x=cos^4\frac{x}{2}\left(1+tan^2\frac{x}{2}\right)\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}cosx\ge0\\cos^2x=cos^4\frac{x}{2}.\frac{1}{cos^2\frac{x}{2}}\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}cosx\ge0\\cos^2x=cos^2\frac{x}{2}\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}cosx\ge0\\cos^2x=\frac{1}{2}+\frac{1}{2}cosx\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}cosx\ge0\\2cos^2x-cosx-1=0\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}cosx=1\\cosx=-\frac{1}{2}< 0\left(l\right)\end{matrix}\right.\)
\(\Rightarrow x=k2\pi\)
