a) \(\overrightarrow {AB} + \overrightarrow {CD} + \overrightarrow {EA} = \overrightarrow {EA} + \overrightarrow {AB} + \overrightarrow {CD} = \overrightarrow {EB} + \overrightarrow {CD} \)

\( = \overrightarrow {ED} + \overrightarrow {DB} + \overrightarrow {CB} + \overrightarrow {BD} \)

\( = \overrightarrow {ED} + \overrightarrow {CB} + \overrightarrow {DB} + \overrightarrow {BD} \)

\( = \overrightarrow {ED} + \overrightarrow {CB} + \overrightarrow 0 \)

\( = \overrightarrow {ED} + \overrightarrow {CB} \).

b) \(\overrightarrow {AC} + \overrightarrow {CD} - \overrightarrow {EC} \)

\( = \overrightarrow {AC} + \overrightarrow {CD} + \overrightarrow {CE} \)

\( = \overrightarrow {AE} + \overrightarrow {CD} \)

\( = \overrightarrow {AE} + \overrightarrow {CB} + \overrightarrow {BD} \)

\( = \overrightarrow {AE} + \overrightarrow {CB} - \overrightarrow {DB} \)

\( = \overrightarrow {AE} - \overrightarrow {DB} + \overrightarrow {CB} \).