$S= \left \{\overrightarrow{AB}; \overrightarrow{AC}; \overrightarrow{AO}; \overrightarrow{AD}; \overrightarrow{BC}; \overrightarrow{BO}; \overrightarrow{BD}; \overrightarrow{CO}; \overrightarrow{CD}; \overrightarrow{DO}; \\\overrightarrow{BA}; \overrightarrow{CA}; \overrightarrow{OA}; \overrightarrow{DA}; \overrightarrow{CB}; \overrightarrow{OB}; \overrightarrow{DB}; \overrightarrow{OC}; \overrightarrow{DC}: \overrightarrow{OD} \right \}$
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: $\overrightarrow{OB}; \overrightarrow{DO}$