$3$\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{\frac{\hspace{2}1\hspace{2}}{\hspace{2}2\hspace{2}} \{ \cos $ {\alpha}+{\beta} $ +\cos $ {\alpha}- {\beta} $ \} }$ ※

$3$\displaystyle{\begin{array}{lll}=\frac{\hspace{2}1\hspace{2}}{\hspace{2}2\hspace{2}} \{\begin{array}{lll}\cos $ \frac{\hspace{2} x+y \hspace{2}}{\hspace{2}2\hspace{2}}+\frac{\hspace{2} x- y \hspace{2}}{\hspace{2}2\hspace{2}} $ & \vspace{6}\\ +\cos $ \frac{\hspace{2} x+y \hspace{2}}{\hspace{2}2\hspace{2}}- \frac{\hspace{2} x- y \hspace{2}}{\hspace{2}2\hspace{2}} $ & \vspace{6}\\ \end{array}\} & \vspace{6}\\ =\frac{\hspace{2}1\hspace{2}}{\hspace{2}2\hspace{2}} \{ \cos x+\cos y \} & \vspace{6}\\ \end{array}}$

$3$\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}}}$