Trương vĩnh siêu's Profile

\[\begin{array}{l}\left( {\cos x + \cos 3x} \right) + \cos 2x = 0\\ \Leftrightarrow 2\cos 2x.\cos x + \cos 2x = 0\\ \Leftrightarrow \cos 2x\left( {2.\cos x + 1} \right) = 0\\ \Leftrightarrow \left[ \begin{array}{l}\cos 2x = 0\\2.\cos x + 1\end{array} \right. \Leftrightarrow ...tu\,\,lam\,\,tiep\,\,nhe!\end{array}\]