加法定理のうち二つの式を辺々引くまたは辺々足すと,積の公式が導ける. #tex{{{ \sin x \sin y &= \frac{1}{2}\{\cos(x-y) - \cos(x+y)\} \\ \sin x \cos y &= \frac{1}{2}\{\sin(x+y) + \sin(x-y)\} \\ \cos x \sin y &= \frac{1}{2}\{\sin(x+y) - \sin(x-y)\} \\ \cos x \cos y &= \frac{1}{2}\{\cos(x-y) + \cos(x+y)\} }}}