和積の公式

$3$\displaystyle{\sin x+\sin y=}$

$3$\displaystyle{- 2\sin \frac{\hspace{2} x+y \hspace{2}}{\hspace{2}2\hspace{2}}\sin \frac{\hspace{2} x- y \hspace{2}}{\hspace{2}2\hspace{2}}}$ ⇒　

$3$\displaystyle{\sin x- \sin y=}$

$3$\displaystyle{2\cos \frac{\hspace{2} x+y \hspace{2}}{\hspace{2}2\hspace{2}}\sin \frac{\hspace{2} x- y \hspace{2}}{\hspace{2}2\hspace{2}}}$ ⇒　

$3$\displaystyle{\cos x+\cos y=}$

$3$\displaystyle{2\cos \frac{\hspace{2} x+y \hspace{2}}{\hspace{2}2\hspace{2}}\cos \frac{\hspace{2} x- y \hspace{2}}{\hspace{2}2\hspace{2}}}$ ⇒　

$3$\displaystyle{\cos x- \cos y=}$

$3$\displaystyle{- 2\sin \frac{\hspace{2} x+y \hspace{2}}{\hspace{2}2\hspace{2}}\sin \frac{\hspace{2} x- y \hspace{2}}{\hspace{2}2\hspace{2}}}$ ⇒　