Cho 2 vectơ 

A

- Hướng dẫn giải

Chọn A

|$\overrightarrow{a}$+$\overrightarrow{b}$| = $\sqrt{{\left(\overrightarrow{a}+\overrightarrow{b}\right)}^2}$ = $\sqrt{{\overrightarrow{a}}^2+2\overrightarrow{a}.\overrightarrow{b}+{\overrightarrow{b}}^2}$

= $\sqrt{{\left|\overrightarrow{a}\right|}^2+2.\left|\overrightarrow{a}\right|\left|\overrightarrow{b}\right|.\cos \left(\overrightarrow{a};\overrightarrow{b}\right)+{\left|\overrightarrow{b}\right|}^2}$

= $\sqrt{4^2+5^2+2.4.5.\cos {120}^0}$ = $\sqrt{21}$