

Cho [tex]\triangle ABC[/tex], trung tuyến AM . D là trung điểm AM. Chứng minh:
a/ 2[tex]\underset{DA}{\rightarrow}+\underset{DB}{\rightarrow}+\underset{DC}{\rightarrow} = \underset{0}{\rightarrow}[/tex]
b/ [tex]2\underset{OA}{\rightarrow}+\underset{OB}{\rightarrow}+\underset{OC}{\rightarrow} = 4\underset{OD}{\rightarrow}[/tex]
a/ 2[tex]\underset{DA}{\rightarrow}+\underset{DB}{\rightarrow}+\underset{DC}{\rightarrow} = \underset{0}{\rightarrow}[/tex]
b/ [tex]2\underset{OA}{\rightarrow}+\underset{OB}{\rightarrow}+\underset{OC}{\rightarrow} = 4\underset{OD}{\rightarrow}[/tex]