$\begin{array}{l}
1/\overrightarrow {AB} + \overrightarrow {CD} = \overrightarrow {AD} + \overrightarrow {CB} \\
\Leftrightarrow \overrightarrow {AB} - \overrightarrow {AD} = \overrightarrow {CB} - \overrightarrow {CD} \\
\Leftrightarrow \overrightarrow {DB} = \overrightarrow {DB} \left( {dung} \right)\\
2/\\
\overrightarrow {OA} + \overrightarrow {OB} + \overrightarrow {OC} + \overrightarrow {OD} = \left( {\overrightarrow {OA} + \overrightarrow {OC} } \right) + \left( {\overrightarrow {OB} + \overrightarrow {OD} } \right) = \overrightarrow 0 + \overrightarrow 0 = \overrightarrow 0
\end{array}$
