\(\cos{x}\cos{y} = \frac{1}{2}(\cos{(x+y)}+\cos{(x-y)})\)
\(\sin{x}\sin{y} = \frac{1}{2}(\cos{(x-y)}-\cos{(x+y)})\)
\(\sin{x}\cos{y} = \frac{1}{2}(\sin{(x+y)}+\cos{(x-y)})\)