Toán Lớp 11: Giải cos3x + cos2x =0
Giúp mk vs ạ
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Toán Lớp 11: Giải cos3x + cos2x =0
Giúp mk vs ạ
Comments ( 2 )
x = \dfrac{\pi }{5} + \dfrac{{k2\pi }}{5}\\
x = \pi + k2\pi
\end{array} \right.\)
\cos 3x + \cos 2x = 0\\
\to 2\cos \left( {\dfrac{{5x}}{2}} \right)\cos \left( {\dfrac{x}{2}} \right) = 0\\
\to \left[ \begin{array}{l}
\cos \left( {\dfrac{{5x}}{2}} \right) = 0\\
\cos \left( {\dfrac{x}{2}} \right) = 0
\end{array} \right.\\
\to \left[ \begin{array}{l}
\dfrac{{5x}}{2} = \dfrac{\pi }{2} + k\pi \\
\dfrac{x}{2} = \dfrac{\pi }{2} + k\pi
\end{array} \right.\\
\to \left[ \begin{array}{l}
x = \dfrac{\pi }{5} + \dfrac{{k2\pi }}{5}\\
x = \pi + k2\pi
\end{array} \right.\left( {k \in Z} \right)
\end{array}\)