Cho 2 vectơ 

B

- Hướng dẫn giải

Chọn B.

Ta có: $\left|\overrightarrow{a}+\overrightarrow{b}\right|=\sqrt{{\left(\overrightarrow{a}+\overrightarrow{b}\right)}^2}=\sqrt{{\overrightarrow{a}}^2+2\overrightarrow{a}\overrightarrow{b}+{\overrightarrow{b}}^2}=\sqrt{{\left|\overrightarrow{a}\right|}^2+2\overrightarrow{a}\overrightarrow{b}+{\left|\overrightarrow{b}\right|}^2}=\sqrt{4^2+2.4.5.\left(-0,5\right)+5^2}=\sqrt{61}$