Cho các vectơ

A

- Hướng dẫn giải

Chọn A

Ta có: $\overrightarrow{a}\left(1;-2\right)$; $\overrightarrow{b}\left(-2;-6\right)$

nên $\overrightarrow{a}.\overrightarrow{b}$ = 1.(–2) + (–2).(–6) = 10

|$\overrightarrow{a}$| = $\sqrt{1^2+{\left(-2\right)}^2}=\sqrt{5}$

|$\overrightarrow{b}$| = $\sqrt{{\left(-2\right)}^2+{\left(-6\right)}^2}=\sqrt{40}$

Suy ra cos($\overrightarrow{a};\overrightarrow{b}$) = $\frac{\overrightarrow{a}.\overrightarrow{b}}{\left|\overrightarrow{a}\right|.\left|\overrightarrow{b}\right|}=\frac{10}{\sqrt{5}.\sqrt{40}}=\frac{\sqrt{2}}{2}$

=> ($\overrightarrow{a};\overrightarrow{b}$) = ${45}^0$