$\vec{c}=(x_{1}; \ y_{1})$ и $\vec{d}=(x_{2}; \ y_{2})$ $\vec{c} \cdot \vec{d} = x_{1} \cdot x_{2} + y_{1} \cdot y_{2}$

$k \cdot \vec{c} = (k \cdot x_{1}; \ k \cdot y_{1})$