Em tự vẽ hình nhé
\(\begin{array}{l}
a)\,\overrightarrow {OA} - \overrightarrow {OD} + \overrightarrow {BC} = \overrightarrow {DA} + \overrightarrow {BC} = \overrightarrow {CB} + \overrightarrow {BC} = \overrightarrow 0 \\
\left( {do\,\,ABCD\,la\,hbh \Rightarrow \overrightarrow {DA} = \overrightarrow {CB} } \right)\\
b)\overrightarrow {BA} - \overrightarrow {OA} = \overrightarrow {BA} + \overrightarrow {AO} = \overrightarrow {BO} = \overrightarrow {OD} \\
(Voi\,O\,la\,giao\,diem\,hai\,duong\,cheo\,AC,BD)
\end{array}\)