\(Pt\Leftrightarrow 5(1-cos^{2}2x)+3.cos2x+3=0\) (1)
Đặt \(A=cos2x, -1\leq A\leq 1\), Ta có:
\((1)\Leftrightarrow -5A^{2}+3A+8=0\)
\(\Rightarrow {\begin{align*}
A&=\frac{8}{5}\\
A&= -1
\end{align*}}{}\) (loại \(A=\frac{8}{5}\))
Ta có:
\(A=-1 \Rightarrow cos2x=-1\Rightarrow x=\frac{\pi }{2}+k\pi ,\forall k\in \mathbb{Z}\)