\(\overrightarrow {AB} + \overrightarrow {BC} + \overrightarrow {CD} = \overrightarrow {AE} - \overrightarrow {DE} \)\( \Leftrightarrow \overrightarrow {AC} + \overrightarrow {CD} = \overrightarrow {AE} + \overrightarrow {ED} \) \( \Leftrightarrow \overrightarrow {AD} = \overrightarrow {AD} \)
