Question

\(\cos\left(x+\frac{\pi}{3}\right)+\cos x=\frac{3}{2}-4\sin\left(\frac{x}{2}\right)\cdot\sin\left(\frac{x}{2}+\frac{\pi}{6}\right)\)

Answer 1

\(\Leftrightarrow1-2sin^2\left(\frac{x}{2}+\frac{\pi}{6}\right)+1-2sin^2\frac{x}{2}=\frac{3}{2}-4sin\left(\frac{x}{2}\right)sin\left(\frac{x}{2}+\frac{\pi}{6}\right)\)

\(\Leftrightarrow\left[sin\left(\frac{x}{2}\right)-sin\left(\frac{x}{2}+\frac{\pi}{6}\right)\right]^2=\frac{1}{4}\)

\(\Leftrightarrow4cos^2\left(\frac{x}{2}+\frac{\pi}{12}\right).sin^2\left(\frac{\pi}{12}\right)=\frac{1}{4}\)

\(\Leftrightarrow cos^2\left(\frac{x}{2}+\frac{\pi}{12}\right)=\frac{1}{16sin^2\left(\frac{\pi}{12}\right)}=\frac{2+\sqrt{3}}{4}\)

\(\Leftrightarrow1+cos\left(x+\frac{\pi}{6}\right)=\frac{2+\sqrt{3}}{2}\)

\(\Leftrightarrow cos\left(x+\frac{\pi}{6}\right)=\frac{\sqrt{3}}{2}\)

\(\Leftrightarrow...\)