\(y=2\left(\frac{1}{2}-\frac{1}{2}cos2x\right)+cos^22x=cos^22x-cos2x+1\)
\(=\left(cos2x-\frac{1}{2}\right)^2+\frac{3}{4}\ge\frac{3}{4}\)
\(\Rightarrow y_{min}=\frac{3}{4}\) khi \(cos2x=\frac{1}{2}\)
\(y=cos^22x-2cos2x+cos2x-2+3\)
\(y=\left(cos2x-2\right)\left(cos2x+1\right)+3\)
Do \(-1\le cos2x\le1\Rightarrow\left\{{}\begin{matrix}cos2x-2< 0\\cos2x+1\ge0\end{matrix}\right.\) \(\Rightarrow\left(cos2x-2\right)\left(cos2x+1\right)\le0\)
\(\Rightarrow y\le3\Rightarrow y_{max}=3\) khi \(cos2x=-1\)
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