Đáp án:
$\begin{array}{l}
{\rm{1) cos}}\left( {\overrightarrow a ,\overrightarrow b } \right) = \frac{{\overrightarrow a .\overrightarrow b }}{{\left| {\overrightarrow a } \right|.\left| {\overrightarrow b } \right|}} = \frac{{ - 2.4 + 3.1}}{{\sqrt {{2^2} + {3^2}} .\sqrt {{4^2} + 1} }} = \frac{{ - 5}}{{\sqrt {221} }}\\
2){\rm{cos}}\left( {\overrightarrow a ,\overrightarrow i } \right) = \frac{{\overrightarrow a .\overrightarrow i }}{{\left| {\overrightarrow a } \right|.\left| {\overrightarrow i } \right|}} = \frac{{ - 2.1 + 3.0}}{{\sqrt {{2^2} + {3^2}} .\sqrt 1 }} = \frac{{ - 2}}{{\sqrt {13} }}\\
3){\rm{cos}}\left( {\overrightarrow j ,\overrightarrow b } \right) = \frac{{\overrightarrow j .\overrightarrow b }}{{\left| {\overrightarrow j } \right|.\left| {\overrightarrow b } \right|}} = \frac{{0.4 + 1.1}}{{\sqrt 1 .\sqrt {{4^2} + 1} }} = \frac{1}{{\sqrt {17} }}
\end{array}$
các câu còn lại tương tự nhé
