Ta có: \(2\overrightarrow{MN}=\overrightarrow{MN}+\overrightarrow{MN}\)
\(=\overrightarrow{MA}+\overrightarrow{AC}+\overrightarrow{CN}+\overrightarrow{MB}+\overrightarrow{BD}+\overrightarrow{DN}\)
\(=\overrightarrow{MA}+\overrightarrow{MB}+\overrightarrow{DN}+\overrightarrow{CN}+\overrightarrow{AC}+\overrightarrow{BD}\)
\(=\overrightarrow{0}+\overrightarrow{0}+\overrightarrow{AC}+\overrightarrow{BD}=\overrightarrow{AC}+\overrightarrow{BD}\)
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