6. Có 4 vecto là \(\overrightarrow{BA};\overrightarrow{CA};\overrightarrow{DA};\overrightarrow{EA}\)
7. \(P=cos0+cos180^0+\left(cos20+cos160\right)+...+\left(cos80+cos100\right)\)
\(=\left(cos0-cos0\right)+\left(cos20-cos20\right)+...+\left(cos80-cos80\right)=0+0+...+0=0\)
8. \(\overrightarrow{BC}-\overrightarrow{AB}=\overrightarrow{BC}+\overrightarrow{BA}=\overrightarrow{BD}\)