Ta có:
$\underset{AB}{\rightarrow}$. $\underset{CD}{\rightarrow}$ + $\underset{AC}{\rightarrow}$.$\underset{DB}{\rightarrow}$ +$\underset{AD}{\rightarrow}$.$\underset{BC}{\rightarrow}$
= $\underset{AB}{\rightarrow}$. ($\underset{AD}{\rightarrow}$-$\underset{AC}{\rightarrow}$) +$\underset{AC}{\rightarrow}$. ($\underset{AB}{\rightarrow}$-$\underset{AD}{\rightarrow}$) +$\underset{AD}{\rightarrow}$. ($\underset{AC}{\rightarrow}$ -$\underset{AB}{\rightarrow}$)
= 0