

[tex]2\underset{MN}{\rightarrow}=\underset{AC}{\rightarrow}+\underset{BD}{\rightarrow}=\underset{BC}{\rightarrow}+\underset{AD}{\rightarrow}[/tex]
chứng minh [tex]2\underset{MN}{\rightarrow}=\underset{AC}{\rightarrow}+\underset{BD}{\rightarrow}[/tex]
chứng minh [tex]2\underset{MN}{\rightarrow}=\underset{AC}{\rightarrow}+\underset{BD}{\rightarrow}[/tex]