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设△OAB的顶点坐标为O(0,0),A(x<sub>1</sub>,y<sub>1</sub>),B(x<sub>2</sub>,y<sub>2</sub>)(按逆时针方向排列),则x<sub>1</sub>y<sub>1</sub>-x<sub>2</sub>y<sub>1</sub>=|x<sub>1