Ta có:
$\underset{AC}{\rightarrow}$ + $\underset{BD}{\rightarrow}$
= ($\underset{AO}{\rightarrow}$+$\underset{OC}{\rightarrow}$) + ($\underset{BO}{\rightarrow}$ + $\underset{OD}{\rightarrow}$)
= ($\underset{AO}{\rightarrow}$ + $\underset{BO}{\rightarrow}$) + ($\underset{OC}{\rightarrow}$ + $\underset{OD}{\rightarrow}$)
= $\underset{0}{\rightarrow}$ + ($\underset{OC}{\rightarrow}$ + $\underset{OD}{\rightarrow}$)
= $\underset{OC}{\rightarrow}$ + $\underset{OD}{\rightarrow}$