Cho tam giác đều ABC cạnh 4. Vectơ \( - \frac{1}{2}\overrightarrow {BC} \) có độ dài là.

A. 5 cm;

B. 7 cm;

C. 9 cm;

D. 11 cm.

A. \(\overrightarrow {MA} + \overrightarrow {MB} + \overrightarrow {MC} = \overrightarrow {MG} \);    

B. \(\overrightarrow {MA} + \overrightarrow {MB} + \overrightarrow {MC} = 2\overrightarrow {MG} \);

C. \(\overrightarrow {MA} + \overrightarrow {MB} + \overrightarrow {MC} = 3\overrightarrow {MG} \);  

D. \(\overrightarrow {MA} + \overrightarrow {MB} + \overrightarrow {MC} = 4\overrightarrow {MG} \).

A. sin (180° – α) = – sin α;

B. cos (180° – α) = – cos α;

C. tan (180° – α) = tan α;

D. cot (180° – α) = cot α.

A. \(\overrightarrow u = \left( {2; - 1} \right)\);

B. \(\overrightarrow u = \left( { - 2;1} \right)\);  

C. \(\overrightarrow u = \left( {2;1} \right)\);

D. \(\overrightarrow u = \left( { - 2; - 1} \right)\).

A. A ∪ B = [– 5; 1);             

B. A ∪ B = [– 5; 3];              

C. A ∪ B = (– 3; 1);              

D. A ∪ B = (– 3; 3].

A. \(\overrightarrow a .\overrightarrow b = \left| {\overrightarrow a } \right|.\left| {\overrightarrow b } \right|.\sin \left( {\overrightarrow a ,\overrightarrow b } \right)\);

B. \(\overrightarrow a .\overrightarrow b = \left| {\overrightarrow a } \right|.\left| {\overrightarrow b } \right|.\cos \left( {\overrightarrow a ,\overrightarrow b } \right)\);

C. \(\overrightarrow a .\overrightarrow b = - \left| {\overrightarrow a } \right|.\left| {\overrightarrow b } \right|.\cos \left( {\overrightarrow a ,\overrightarrow b } \right)\);                                            

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