$S= \left \{\overrightarrow{AB}; \overrightarrow{AC}; \overrightarrow{AO}; \overrightarrow{AD}; \overrightarrow{BC}; \overrightarrow{BO}; \overrightarrow{BD}; \overrightarrow{CO}; \overrightarrow{CD}; \overrightarrow{DO}; \\\overrightarrow{BA}; \overrightarrow{CA}; \overrightarrow{OA}; \overrightarrow{DA}; \overrightarrow{CB}; \overrightarrow{OB}; \overrightarrow{DB}; \overrightarrow{OC}; \overrightarrow{DC}: \overrightarrow{OD} \right \}$
Các nhóm:
+ nhóm 1: $\overrightarrow{AB}; \overrightarrow{DC}$
+ nhóm 2: $\overrightarrow{BA}; \overrightarrow{CD}$
+ nhóm 3: $\overrightarrow{AD}; \overrightarrow{BC}$
+ nhóm 4: $\overrightarrow{DA}; \overrightarrow{CB}$
+ nhóm 5: $\overrightarrow{AO}; \overrightarrow{OC}$
+ nhóm 6: $\overrightarrow{OA}; \overrightarrow{CO}$
+ nhóm 7: $\overrightarrow{BO}; \overrightarrow{OD}$
+ nhóm 8: $\overrightarrow{OB}; \overrightarrow{DO}$