Question

Giải phương trình sau:

sin2x + sinx - 2sin²x + cosx +1 =0

Answer 1

sin2x + 1 - 2sin²x + sinx + cosx = 0

⇔ sin2x + cos2x + sinx + cosx = 0

⇔ \(\sqrt{2}sin\left(2x+\dfrac{\pi}{4}\right)+\sqrt{2}sin\left(x+\dfrac{\pi}{4}\right)=0\)

⇔ \(sin\left(2x+\dfrac{\pi}{4}\right)+sin\left(x+\dfrac{\pi}{4}\right)=0\)

⇔ \(2sin\left(\dfrac{3x}{2}+\dfrac{\pi}{4}\right).cos\dfrac{x}{2}=0\)

⇔ \(\left[{}\begin{matrix}sin\left(3x+\dfrac{\pi}{4}\right)=0\\cos\dfrac{x}{2}=0\end{matrix}\right.\)

⇔ \(\left[{}\begin{matrix}x=-\dfrac{\pi}{12}+k.\dfrac{\pi}{3}\\x=\pi+k.2\pi\end{matrix}\right.\) , k ∈ Z