Do O là tâm hình bình hành
\(\Rightarrow\left\{{}\begin{matrix}\overrightarrow{OA}+\overrightarrow{OC}=\overrightarrow{0}\\\overrightarrow{OB}+\overrightarrow{OD}=\overrightarrow{0}\end{matrix}\right.\)
\(\Rightarrow\overrightarrow{IA}+\overrightarrow{IC}=\overrightarrow{IO}+\overrightarrow{OA}+\overrightarrow{IO}+\overrightarrow{OC}=2\overrightarrow{IO}\) (1)
\(\overrightarrow{IB}+\overrightarrow{ID}=\overrightarrow{IO}+\overrightarrow{OB}+\overrightarrow{IO}+\overrightarrow{OD}=2\overrightarrow{IO}\) (2)
(1);(2) \(\Rightarrow\overrightarrow{IA}+\overrightarrow{IC}=\overrightarrow{IB}+\overrightarrow{ID}\)
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