[Toán 10] lượng giác

L

levietdung1998

\[\begin{array}{l}
\cos \left( {x - \frac{\pi }{3}} \right)\cos \left( {\frac{\pi }{4} + x} \right) + \cos \left( {\frac{\pi }{6} + x} \right)\cos \left( {x + \frac{{3\pi }}{6}} \right)\\
= \frac{1}{2}\left[ {\cos \left( {2x - \frac{\pi }{{12}}} \right) + \cos \frac{{7\pi }}{{12}}} \right] + \frac{1}{2}\left[ {\cos \left( {2x + \frac{{2\pi }}{3}} \right) + \cos \frac{\pi }{3}} \right]\\
= \frac{1}{2}\left[ {2\cos \frac{{4x + \frac{{7\pi }}{{12}}}}{2}\cos \frac{{\frac{{3\pi }}{4}}}{2} + 2\cos \frac{{\frac{{11\pi }}{{12}}}}{2}\cos \frac{{\frac{\pi }{4}}}{2}} \right]\\
= \cos \left( {2x + \frac{{7\pi }}{{24}}} \right)\cos \frac{{3\pi }}{8} + \cos \frac{{11\pi }}{{24}}\cos \frac{\pi }{8}
\end{array}\]

"Bài dự thi event box 10 "
 
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