a) \(\overrightarrow u = \overrightarrow {AB} + \overrightarrow {DC} + \overrightarrow {BD} + \overrightarrow {CA} \)\( = \left( {\overrightarrow {AB} + \overrightarrow {BD} } \right) + \left( {\overrightarrow {DC} + \overrightarrow {CA} } \right)\) \( = \overrightarrow {AD} + \overrightarrow {DA} = \overrightarrow {AA} = \overrightarrow 0 \)
b) \(\overrightarrow v = \overrightarrow {AB} + \overrightarrow {CD} + \overrightarrow {BC} + \overrightarrow {DA} \)\( = \left( {\overrightarrow {DA} + \overrightarrow {AB} } \right) + \left( {\overrightarrow {BC} + \overrightarrow {CD} } \right)\) \( = \overrightarrow {DB} + \overrightarrow {BD} = \overrightarrow {DD} = \overrightarrow 0 \)
