Ta có công thức:

$\overrightarrow{u} \cdot \overrightarrow{v} = |\overrightarrow{u}| \cdot |\overrightarrow{v}| \cdot cos(\overrightarrow{u}, \overrightarrow{v})$

Theo đề bài:

$\overrightarrow{u} \cdot \overrightarrow{v} = -|\overrightarrow{u}| \cdot |\overrightarrow{v}|$

$\Leftrightarrow |\overrightarrow{u}| \cdot |\overrightarrow{v}| \cdot cos(\overrightarrow{u}, \overrightarrow{v}) = -|\overrightarrow{u}| \cdot |\overrightarrow{v}|$

$\Leftrightarrow cos(\overrightarrow{u}, \overrightarrow{v}) = -1$

$\Leftrightarrow (\overrightarrow{u}, \overrightarrow{v}) = 180°$

`-> bbA`