Cho tam giác ABC với

- Hướng dẫn giải

Ta có $\left\{\begin{array}{l}\left(\overrightarrow{AB},\overrightarrow{BC}\right)={180}^0-\widehat{ABC}\\ \left(\overrightarrow{BC},\overrightarrow{CA}\right)={180}^0-\widehat{BCA}\end{array}\right.$

$\Rightarrow \left(\overrightarrow{AB},\overrightarrow{BC}\right)+\left(\overrightarrow{BC},\overrightarrow{CA}\right)={360}^0-\left(\widehat{ABC}+\widehat{BCA}\right)$

$={360}^0-\left({180}^0-\widehat{BAC}\right)={360}^0-{180}^0+{60}^0={240}^0.$

ĐÁP ÁN D