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cho tam giác ABC. lấy M, N, P thỏa mãn: $\overrightarrow{MB}=3\overrightarrow{MC}$, $\overrightarrow{NA}+3\overrightarrow{NC}=\overrightarrow{0}$, $\overrightarrow{PA}+\overrightarrow{PB}=\overrightarrow{0}$. tìm $\overrightarrow{MP}$, $\overrightarrow{MN}$ theo $\overrightarrow{AB}$,$\overrightarrow{AC}$. suy ra M, N, P thẳng hàng
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