Cho các vectơ \(\overrightarrow a = \left( {1; - 2} \right),\,\,\overrightarrow b  = \left( { - 2; - 6} \right)\). Khi đó góc giữa chúng là

\(16\).

\(26\).

\(36\).

\( - 16\).

\(\overrightarrow a .\overrightarrow b  = \left| {\overrightarrow a } \right|.\left| {\overrightarrow b } \right|\).

\(\overrightarrow a .\overrightarrow b  = 0\).

\(\overrightarrow a .\overrightarrow b  =  - 1\).

\(\overrightarrow a .\overrightarrow b =  - \left| {\overrightarrow a } \right|.\left| {\overrightarrow b } \right|\).

$1$

$2$

$3$

$4$

\(3\).

\(4\).

\(1\).

\(2\).

\(\overrightarrow {OA} .\overrightarrow {OB}  = 0\).

\(\overrightarrow {OA} .\overrightarrow {OC}  = \dfrac{1}{2}\overrightarrow {OA} .\overrightarrow {AC} \).

\(\overrightarrow {AB} .\overrightarrow {AC}  = \overrightarrow {AB} .\overrightarrow {CD} \).

\(\overrightarrow {AB} .\overrightarrow {AC}  = \overrightarrow {AC} .\overrightarrow {AD} \).

\(\cos B = \dfrac{1}{{\sqrt 3 }}\).

\(\sin C = \dfrac{{\sqrt 3 }}{2}\).

\(\cos C = \dfrac{1}{2}\).

\(\sin B = \dfrac{1}{2}\)