lượng giác khó mọi người giúp mình nha ^^

N

nttthn_97

$A=cos\frac{\pi}{7}.cos(\pi -\frac{4\pi}{7}).cos(\pi- \frac{2\pi}{7})$

$=cos\frac{\pi}{7}.-cos\frac{4\pi}{7}.-cos\frac{2\pi}{7}$

$A.sin\frac{\pi}{7}=\frac{1}{2}.sin\frac{2\pi}{7}.cos\frac{2\pi}{7}.cos\frac{4\pi}{7}$

$=\frac{1}{4}.sin\frac{4\pi}{7}.cos\frac{4\pi}{7}$

$=\frac{1}{8}sin\frac{8\pi}{7}=-\frac{1}{8}.sin\frac{\pi}{7}$

[TEX]\Rightarrow[/TEX]$A=\frac{-1}{8}$
 
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