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Nếu ý của bạn là Giải phương trình : $2\cos^2x-\cos x-\sin x-1=0$
thì PT $\Leftrightarrow 2\cos^2x-1+\cos x-\sin x=0$
$\Leftrightarrow 2\cos^2x-(\cos^2 x+ \sin^2 x)+\cos x-\sin x=0$
$\Leftrightarrow \cos^2 x-\sin^2 x+\cos x-\sin x=0$
$\Leftrightarrow(\cos x-\sin x)(\cos x+\sin x+1)=0$
$\Leftrightarrow \left[ {\begin{matrix} \cos x-\sin x=0\\\cos x+\sin x=-1\end{matrix}} \right.$
$\Leftrightarrow \left[ {\begin{matrix} \sin (x- \frac{\pi}{4})=0\\\sin (x+ \frac{\pi}{4})=\frac{-1}{\sqrt 2}\end{matrix}} \right.$
$\Leftrightarrow \left[ {\begin{matrix} x= \frac{\pi}{4}+ k\pi\\x=- \frac{\pi}{2}+ k2\pi\\x=\pi+ k2\pi\end{matrix}} \right.$
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