Cho đường thẳng \({d_1}:x +...

C

- Hướng dẫn giải

\(\begin{array}{l}\left\{ \begin{array}{l}{d_1}:x + 2y - 7 = 0 \to {{\vec n}_1} = \left( {1;2} \right)\\{d_2}:2x - 4y + 9 = 0 \to {{\vec n}_2} = \left( {1; - 2} \right)\end{array} \right.\\\varphi = \left( {{d_1};{d_2}} \right) \Rightarrow \cos \varphi = \frac{{\left| {1 - 4} \right|}}{{\sqrt {1 + 4} .\sqrt {1 + 4} }} = \frac{3}{5}.\end{array}\)