\[\begin{array}{l}
BD = AC = \sqrt {A{B^2} + A{D^2}} = \sqrt {4{a^2} + 16{a^2}} = 2a\sqrt 5 .\\
+ )\,\,\overrightarrow {OC} - \overrightarrow {OB} - \overrightarrow {AB} = \overrightarrow {BC} - \overrightarrow {AB} = \overrightarrow {AD} - \overrightarrow {AB} = \overrightarrow {BD} \\
\Rightarrow \left| {\overrightarrow {OC} - \overrightarrow {OB} - \overrightarrow {AB} } \right| = \left| {\overrightarrow {BD} } \right| = BD = 2a\sqrt 5 .\\
+ )\,\,\,\overrightarrow {AD} - \overrightarrow {CD} - \overrightarrow {BA} = \overrightarrow {AD} + \overrightarrow {DC} + \overrightarrow {DC} = \overrightarrow {AD} + 2\overrightarrow {DC} .
\end{array}\]
