\(2cosx+\sqrt{2}=0\Leftrightarrow cosx=-\dfrac{\sqrt{2}}{2}\)
\(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{3\pi}{4}+k2\pi\\x=-\dfrac{3\pi}{4}+n2\pi\end{matrix}\right.\)
\(-6< x< 6\Rightarrow\left\{{}\begin{matrix}-6< \dfrac{3\pi}{4}+k2\pi< 6\\-6< -\dfrac{3\pi}{4}+n2\pi< 6\end{matrix}\right.\)
\(\Rightarrow\left\{{}\begin{matrix}-1,33< k< 0,58\\-0,58< n< 1,33\end{matrix}\right.\)
\(\Rightarrow\left\{{}\begin{matrix}k=\left\{-1;0\right\}\\n=\left\{0;1\right\}\end{matrix}\right.\) \(\Rightarrow\) có 4 nghiệm
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