a: \(\overrightarrow{AB}+2\overrightarrow{AC}\)
\(=\overrightarrow{AM}+\overrightarrow{MB}+2\left(\overrightarrow{AM}+\overrightarrow{MC}\right)\)
\(=\overrightarrow{AM}+\overrightarrow{MB}+2\overrightarrow{AM}+2\overrightarrow{MC}\)
\(=3\overrightarrow{AM}+\overrightarrow{MB}+2\overrightarrow{MC}=3\cdot\overrightarrow{AM}\)
b:
\(\overrightarrow{MA}-\overrightarrow{MG}+\overrightarrow{MB}-\overrightarrow{MG}+\overrightarrow{MC}-\overrightarrow{MG}\)
\(=\overrightarrow{GA}+\overrightarrow{GB}+\overrightarrow{GC}\)
\(=\overrightarrow{0}\)(do G là trọng tâm của ΔABC)
=>\(\overrightarrow{MA}+\overrightarrow{MB}+\overrightarrow{MC}=3\cdot\overrightarrow{MG}\)
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