Vì ABCD là hình bình hành => $\overrightarrow{BA} =-\overrightarrow{DC}$
=> $\overrightarrow{BA} +\overrightarrow{DC}=\overrightarrow{0}$
Mặt khác: $\overrightarrow{MA} +\overrightarrow{MC}=(\overrightarrow{MB}+\overrightarrow{BA})+(\overrightarrow{MD}+\overrightarrow{DC})$
<=>$\overrightarrow{MA}+\overrightarrow{MC}=\overrightarrow{MB}+\overrightarrow{MD}+\overrightarrow{BA}+\overrightarrow{DC}$
<=> $\overrightarrow{MA} +\overrightarrow{MC}=\overrightarrow{MB}+\overrightarrow{MD}$ (đpcm)