Ta có: $\overrightarrow{u}.\overrightarrow{v}=|\overrightarrow{u}|.|\overrightarrow{v}|.cos(\overrightarrow{u}.\overrightarrow{v})$
a. Nếu $\overrightarrow{u}.\overrightarrow{v}=|\overrightarrow{u}|.|\overrightarrow{v}|$ thì $cos(\overrightarrow{u}.\overrightarrow{v})$=1
=> $(\overrightarrow{u}.\overrightarrow{v})=0^{o}$
Vậy $\overrightarrow{u}, \overrightarrow{v}$ cùng hướng
b. Nếu $\overrightarrow{u}.\overrightarrow{v}=|\overrightarrow{u}|.|\overrightarrow{v}|$ thì $cos(\overrightarrow{u}.\overrightarrow{v})$=-1
=> $(\overrightarrow{u}.\overrightarrow{v})=180^{o}$
Vậy $\overrightarrow{u}, \overrightarrow{v}$ ngược hướng.