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證明: 因為向量AM+MN+ND+DA=0 向量BM+MN+NC+CB=0 二式相加得: 2向量MN+(AM+BM)+(ND+NC)+(DA+CB)=0 又M,N是中點,故向量AM+BM=0,ND+NC=0 所以,2向量MN=-(向量DA+CB) 即:向量MN=1/2(向量AD+BC)
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