题目

用坐标法证明++=0. 答案：解：设A(x1,y1)、B(x2,y2)、C(x3,y3).=(x2-x1,y2-y1),=(x3-x2,y3-y2),=(x1-x3,y1-y3),∴++=(x2-x1,y2-y1)+(x3-x2,y3-y2)+(x1-x3,y1-y3)=(x2-x1+x3-x2+x1-x3,y2-y1+y3-y2+y1-y3)=(0,0).∴++=0.

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