Cho M, N,I là trung điểm AB,CD,MN
Chứng minh: 1) \(\overrightarrow{MN}=\frac{1}{2}\left(\overrightarrow{AC}+\overrightarrow{BD}\right)=\frac{1}{2}\left(\overrightarrow{AD}+\overrightarrow{BC}\right)\)
2)\(\overrightarrow{IA}+\overrightarrow{IB}+\overrightarrow{IC}+\overrightarrow{ID}=\overrightarrow{0}\)
3)\(\overrightarrow{OA}+\overrightarrow{OB}+\overrightarrow{OC}+\overrightarrow{OD}=4\overrightarrow{OI}\forall O\)
4) \(\overrightarrow{MC}+\overrightarrow{MD}+\overrightarrow{NA}+\overrightarrow{NB}=\overrightarrow{0}\)
5) \(\overrightarrow{AD}-\overrightarrow{CD}\Leftrightarrow M\equiv N\)
6) \(\overrightarrow{AD}+\overrightarrow{BD}+\overrightarrow{AC}+\overrightarrow{BC}=4\overrightarrow{MN}\)