Biết rằng cos x cos ( π/3 − x ) cos ( π/3 + x ) = k cos 3 x , khi đó k = ?

\(\begin{array}{l}\cos x\cos \left( {\frac{\pi }{3} - x} \right)\cos \left( {\frac{\pi }{3} + x} \right) = \frac{1}{2}\cos x\left( {\cos \frac{{2\pi }}{3} + \cos 2x} \right)\\ = \frac{1}{2}\cos x\cos 2x - \frac{1}{4}\cos x = \frac{1}{4}(\cos 3x + \cos x) - \frac{1}{4}\cos x = \frac{1}{4}\cos 3x\end{array}\)

Trả lời: 0,25