Підготовка до ЗНО з математики. Алгебра.

$\begin{array}{l}\sin \alpha \sin \beta = \frac{1}{2}(\cos (\alpha - \beta ) + \sin (\alpha - \beta ));\\\cos \alpha \cos \beta = \frac{1}{2}(\cos (\alpha - \beta ) + \cos (\alpha - \beta ));\\\sin \alpha \cos \beta = \frac{1}{2}(\sin (\alpha + \beta ) + \sin (\alpha - \beta )).\end{array}$