[tex]Bonjour;\\\\\\ \textit{8 - 1) } \overrightarrow{AC} + \overrightarrow{DB}= \overrightarrow{AD}+\overrightarrow{DC}+\overrightarrow{DA}+\overrightarrow{AB} \\\\\\ = \overrightarrow{AD}+\overrightarrow{DA}+\overrightarrow{AB}+\overrightarrow{DC} = \overrightarrow{AA}+\overrightarrow{AB}+\overrightarrow{DC} \\\\\\ = \vec 0 + \overrightarrow{AB}+\overrightarrow{DC} = \overrightarrow{AB}+\overrightarrow{DC} .[/tex]
[tex]\textit{10 - 1) } \overrightarrow{MA}+\overrightarrow{MB}+\overrightarrow{MC}+\overrightarrow{MD} [/tex]
[tex]= \overrightarrow{MO}+\overrightarrow{OA}+\overrightarrow{MO}+ \overrightarrow{OB}+\overrightarrow{MO}+\overrightarrow{OC} +\overrightarrow{MO}+ \overrightarrow{OD} \\\\\\ = 4 \overrightarrow{MO}+\overrightarrow{OA}+\overrightarrow{OB}+ \overrightarrow{OC}+\overrightarrow{OD} = 4 \overrightarrow{MO} + \vec 0 = 4 \overrightarrow{MO} \ .[/tex]