2 câu trả lời
$1+\cos2x+\cos x+\cos3x=0$
$\Leftrightarrow 2\cos^2x+2\cos2x.\cos x=0$
$\Leftrightarrow \cos x(\cos x+\cos2x)=0$
$\Leftrightarrow \cos x.2\cos\dfrac{3x}{2}\cos\dfrac{x}{2}=0$
$\cos x=0\Leftrightarrow x=\dfrac{\pi}{2}+k\pi$
$\cos\dfrac{3x}{2}=0\Leftrightarrow \dfrac{\pi}{3}+\dfrac{k2\pi}{3}$
$\cos\dfrac{x}{2}=0\Leftrightarrow x=\pi+k2\pi$
Câu hỏi trong lớp
Xem thêm
