cos3x+cosx=2cosx(sin2x+1)
$\cos 3x+\cos x=2\cos x(\sin 2x+1)$
$\Leftrightarrow 2\cos 2x\cos x=2\cos x(\sin 2x+1)$
$\Leftrightarrow 2\cos x(\cos 2x-\sin 2x-1)=0$
$\Leftrightarrow \cos x=0$ hoặc $\cos 2x-\sin 2x-1=0$(1)
Từ (1) $\Rightarrow  \cos x^{2}-\sin x^{2}=2\sin x\cos x+1$
$\Leftrightarrow -2\sin x(\sin x-\cos x)=0 $ $\Rightarrow \sin x=0 $ hoặc $ \sin x-\cos x=0$
$\Rightarrow  1-2\sin x\cos x=0$ $\Leftrightarrow \sin 2x=1$ 
rồi đó @@@@ –  ♂KKK♂ 03-08-16 10:40 PM
đề bài là cộng không phải là nhân .....cos3xcosx.....cos3xcosx –  shadow night ^.^ 03-08-16 08:44 AM
tìm x chứ s –  ♂KKK♂ 02-08-16 11:22 PM
hình như bạn nhầm đề –  shadow night ^.^ 02-08-16 10:16 PM
làm kiểu j vậy??? –  Băng Băng 02-08-16 10:05 PM

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