a) Ta có: \(\overrightarrow {CO} = \overrightarrow {OA}\)\( \Rightarrow \overrightarrow {CO} - \overrightarrow {OB} = \overrightarrow {OA} - \overrightarrow {OB} = \overrightarrow {BA} .\)
b) Ta có: \(\overrightarrow {BC} = \overrightarrow {AD}\)\( \Rightarrow \overrightarrow {AB} - \overrightarrow {BC} = \overrightarrow {AB} - \overrightarrow {AD} = \overrightarrow {DB} .\)
c) Ta có : \(\left\{ \begin{array}{l}
\overrightarrow {DA} - \overrightarrow {DB} = \overrightarrow {BA} \\
\overrightarrow {OD} - \overrightarrow {OC} = \overrightarrow {CD}
\end{array} \right..\)
Mà \(\overrightarrow {BA} = \overrightarrow {CD} \)\( \Rightarrow \overrightarrow {DA} - \overrightarrow {DB} = \overrightarrow {OD} - \overrightarrow {OC} .\)
d) Ta có: \(\overrightarrow {DA} - \overrightarrow {DB} + \overrightarrow {DC} = \overrightarrow {BA} + \overrightarrow {DC} \)\(= \overrightarrow {BA} + \overrightarrow {AB} = \overrightarrow 0 .\)