Product-To-Sum Identities

	$\displaystyle \sin x \cos y=\frac{1}{2}\big{[}\sin (x+y)+\sin (x-y)\big{]}$
	$\displaystyle \cos x \sin y=\frac{1}{2}\big{[}\sin(x+y)-\sin(x-y)\big{]}$
	$\displaystyle \sin x \sin y=\frac{1}{2}\big{[}\cos(x-y)-\cos(x+y)\big{]}$
	$\displaystyle \cos x \cos y=\frac{1}{2}\big{[}\cos(x+y)+\cos(x-y)\big{]}$