Chứng minh: cos5x.cos3x+ sin7x.sinx = cos2x.cos4x Online chờ gấp~ 08/08/2021 Bởi Julia Chứng minh: cos5x.cos3x+ sin7x.sinx = cos2x.cos4x Online chờ gấp~
$\begin{array}{l} \cos 5x\cos 3x + \sin 7x\sin x\\ = \dfrac{1}{2}\left( {\cos 8x + \cos 2x} \right) + \dfrac{1}{2}\left( {\cos 6x – \cos 8x} \right)\\ = \dfrac{1}{2}\left( {\cos 2x + \cos 6x} \right) = \dfrac{1}{2}.2\cos 4x.\cos 2x = \cos 4x\cos 2x \end{array}$ Bình luận
CHÚC BẠN HỌC TỐT!!! Trả lời: $\cos 5x.\cos 3x+\sin 7x.\sin x\\=\dfrac{1}{2}.(\cos 8x+\cos 2x)+\dfrac{1}{2}.(\cos 6x-\cos 8x)\\=\dfrac{1}{2}(\cos2x+\cos 6x)\\=\dfrac{1}{2}.2.\cos 4x.\cos 2x\\=\cos 2x.\cos 4x.$ Bình luận
$\begin{array}{l} \cos 5x\cos 3x + \sin 7x\sin x\\ = \dfrac{1}{2}\left( {\cos 8x + \cos 2x} \right) + \dfrac{1}{2}\left( {\cos 6x – \cos 8x} \right)\\ = \dfrac{1}{2}\left( {\cos 2x + \cos 6x} \right) = \dfrac{1}{2}.2\cos 4x.\cos 2x = \cos 4x\cos 2x \end{array}$
CHÚC BẠN HỌC TỐT!!!
Trả lời:
$\cos 5x.\cos 3x+\sin 7x.\sin x\\=\dfrac{1}{2}.(\cos 8x+\cos 2x)+\dfrac{1}{2}.(\cos 6x-\cos 8x)\\=\dfrac{1}{2}(\cos2x+\cos 6x)\\=\dfrac{1}{2}.2.\cos 4x.\cos 2x\\=\cos 2x.\cos 4x.$