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About LinkBacksIf you drawstraight lines in the plane consisting of
parallel in one direction,
parallel in different direction ... and
parallel in another direction and no three of the lines meeting at a point, show that the number of intersection points is
![\sum_{i<j} x_{i}x_{j} = \frac{1}{2}\left[m^2 - \sum_{j = 1}^{k} x_{j}^2\right]](http://latex.codecogs.com/png.latex?\sum_{i<j} x_{i}x_{j} = \frac{1}{2}\left[m^2 - \sum_{j = 1}^{k} x_{j}^2\right])
If you draw
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