Với mọi vectơ → a , → b , ta có A. → a ⋅ → b = | → a | ⋅ | → b | ⋅ B. → a ⋅ → b = | → a | ⋅ | → b | ⋅ cos ( → a , → b ) .

\(\vec a \cdot \vec b = |\vec a| \cdot |\vec b| \cdot \)

\(\overrightarrow {\rm{a}} \cdot \overrightarrow {\rm{b}} = |\overrightarrow {\rm{a}} | \cdot |\overrightarrow {\rm{b}} | \cdot \cos (\overrightarrow {\rm{a}} ,\overrightarrow {\rm{b}} ).\)

\(\vec a \cdot \vec b = |\vec a| \cdot |\vec b| \cdot \sin (\vec a,\vec b).\)

\(\overrightarrow {\rm{a}} \cdot \overrightarrow {\rm{b}} = - |\overrightarrow {\rm{a}} | \cdot |\overrightarrow {\rm{b}} | \cdot \)