Ta thấy:
\(\overrightarrow {BC} = \overrightarrow {AD} ,\overrightarrow {CB} = \overrightarrow {DA} \),
\(\overrightarrow {AB} = \overrightarrow {DC} ,\overrightarrow {BA} = \overrightarrow {CD} \),
\(\overrightarrow {OB} = \overrightarrow {DO} ,\overrightarrow {BO} = \overrightarrow {OD} \),
\(\overrightarrow {AO} = \overrightarrow {OC} ,\overrightarrow {CO} = \overrightarrow {OA} \)
