Question
$3\overrightarrow {OD} + \overrightarrow {DA} + \overrightarrow {DB} + \overrightarrow {DC} = $
$ = \overrightarrow {OD} + \overrightarrow {DA} + \overrightarrow {OD} + \overrightarrow {DB} + \overrightarrow {OD} + \overrightarrow {DC} $$ = \overrightarrow {OA} + \overrightarrow {OB} + \overrightarrow {OC} .$
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