Toán Lớp 11: a)cos(3x-20°)=cos7x. Giúp em với ạ
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Toán Lớp 11: a)cos(3x-20°)=cos7x. Giúp em với ạ
Comments ( 2 )
\cos x = \cos y \Leftrightarrow \left[ \begin{array}{l}
x = y + k.360^\circ \\
x = – y + k.360^\circ
\end{array} \right.\,\,\,\left( {k \in Z} \right)\\
\cos \left( {3x – 20^\circ } \right) = \cos 7x\\
\Leftrightarrow \left[ \begin{array}{l}
3x – 20^\circ = 7x + k.360^\circ \\
3x – 20^\circ = – 7x + k.360^\circ
\end{array} \right.\\
\Leftrightarrow \left[ \begin{array}{l}
3x – 20^\circ – 7x = k.360^\circ \\
3x – 20^\circ + 7x = k.360^\circ
\end{array} \right.\\
\Leftrightarrow \left[ \begin{array}{l}
– 4x = 20^\circ + k.360^\circ \\
10x = 20^\circ + k.360^\circ
\end{array} \right.\\
\Leftrightarrow \left[ \begin{array}{l}
x = – 5^\circ + k.90^\circ \\
x = 2^\circ + k.36^\circ
\end{array} \right.\,\,\,\,\left( {k \in Z} \right)
\end{array}\)
x = – 5^\circ + k.90^\circ \\
x = 2^\circ + k.36^\circ
\end{array} \right.\,\,\,\,\left( {k \in Z} \right)\]
\cos x = \cos y \Leftrightarrow \left[ \begin{array}{l}
x = y + k.360^\circ \\
x = – y + k.360^\circ
\end{array} \right.\,\,\,\left( {k \in Z} \right)\\
\cos \left( {3x – 20^\circ } \right) = \cos 7x\\
\Leftrightarrow \left[ \begin{array}{l}
3x – 20^\circ = 7x + k.360^\circ \\
3x – 20^\circ = – 7x + k.360^\circ
\end{array} \right.\\
\Leftrightarrow \left[ \begin{array}{l}
3x – 20^\circ – 7x = k.360^\circ \\
3x – 20^\circ + 7x = k.360^\circ
\end{array} \right.\\
\Leftrightarrow \left[ \begin{array}{l}
– 4x = 20^\circ + k.360^\circ \\
10x = 20^\circ + k.360^\circ
\end{array} \right.\\
\Leftrightarrow \left[ \begin{array}{l}
x = – 5^\circ + k.90^\circ \\
x = 2^\circ + k.36^\circ
\end{array} \right.\,\,\,\,\left( {k \in Z} \right)
\end{array}\)