cos ( d , ( P ) ) = √ 2 3 .

c) Sai.Ta có \(\overrightarrow {{u_d}}  = \left( {2;1; - 1} \right)\), \(\overrightarrow {{n_{\left( P \right)}}}  = \left( {1; - 2; - 2} \right)\).

\(\sin \left( {d,\left( P \right)} \right) = \frac{{\left| {2 \cdot 1 + 1 \cdot \left( { - 2} \right) + \left( { - 1} \right) \cdot \left( { - 2} \right)} \right|}}{{\sqrt {{2^2} + {1^2} + {{\left( { - 1} \right)}^2}}  \cdot \sqrt {{1^2} + {{\left( { - 2} \right)}^2} + {{\left( { - 2} \right)}^2}} }} = \frac{{\sqrt 6 }}{9}\).

\( \Rightarrow \)\({\cos ^2}\left( {d,\left( P \right)} \right) = 1 - {\sin ^2}\left( {d,\left( P \right)} \right) = 1 - {\left( {\frac{{\sqrt 6 }}{9}} \right)^2} = \frac{{25}}{{27}}\).

Suy ra \(\cos \left( {d,\left( P \right)} \right) = \frac{{5\sqrt 3 }}{9}\).