Chứng minh: cos5x.cos3x+ sin7x.sinx = cos2x.cos4x Online chờ gấp~

$\begin{array}{l} \cos 5x\cos 3x + \sin 7x\sin x\\  = \dfrac{1}{2}\left( {\cos 8x + \cos 2x} \right) + \dfrac{1}{2}\left( {\cos 6x - \cos 8x} \right)\\  = \dfrac{1}{2}\left( {\cos 2x + \cos 6x} \right) = \dfrac{1}{2}.2\cos 4x.\cos 2x = \cos 4x\cos 2x \end{array}$