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Trả lời:
$\overrightarrow{MN}\\=\overrightarrow{MA}+\overrightarrow{AD}+\overrightarrow{DN}\\=-\dfrac{1}{4}\overrightarrow{AC}+\overrightarrow{AD}+\dfrac{1}{2}\overrightarrow{AB}\\=-\dfrac{1}{4}.(\overrightarrow{AB}+\overrightarrow{AC})+\overrightarrow{AD}+\dfrac{1}{2}\overrightarrow{AB}\\=-\dfrac{1}{4}\overrightarrow{AB}-\dfrac{1}{4}\overrightarrow{AD}+\overrightarrow{AD}+\dfrac{1}{2}\overrightarrow{AB}\\=\dfrac{1}{4}\overrightarrow{AB}+\dfrac{3}{4}\overrightarrow{AD}.$
