\(\cos^2(x) - \cos(2x) = \sin^2(x)\)

\(\cos(2x) + 2 \sin^2(x) = 1\)

\(\cos(2x) + 1 = 2 \cos^2(x)\)

\((\cos(2x)-1)\cdot \cos(x)  = \sin(2x) \cdot \sin(x)\)