sinx + sin2x = cosx + cos2x
⇔ \(2sin\dfrac{3x}{2}.cos\dfrac{x}{2}=2cos\dfrac{3x}{2}.cos\dfrac{x}{2}\)
⇔ \(\left[{}\begin{matrix}cos\dfrac{x}{2}=0\\sin\dfrac{3x}{2}-cos\dfrac{3x}{2}=0\end{matrix}\right.\)
⇔ \(\left[{}\begin{matrix}cos\dfrac{x}{2}=0\\\sqrt{2}sin\left(\dfrac{3x}{2}-\dfrac{\pi}{4}\right)=0\end{matrix}\right.\)