\(\overrightarrow{AB},\overrightarrow{DC}\) cùng hướng
\(\Rightarrow\left(\overrightarrow{AB},\overrightarrow{DC}\right)=0^o\)
\(\overrightarrow{AD},\overrightarrow{CB}\) ngược hướng
\(\Rightarrow\left(\overrightarrow{AD},\overrightarrow{CB}\right)=180^o\)
Ta gọi \(\overrightarrow{CE}=\overrightarrow{DC}\)
\(\Rightarrow\left(\overrightarrow{CO},\overrightarrow{DC}\right)=\left(\overrightarrow{CO},\overrightarrow{CE}\right)=\widehat{ECO}=180^o-45^o=135^o\)
\(\Rightarrow\left(\overrightarrow{AB},\overrightarrow{DC}\right)+\left(\overrightarrow{AD},\overrightarrow{CB}\right)+\left(\overrightarrow{CO},\overrightarrow{DC}\right)=0^o+180^o+135^o=315^o\)
