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(y2 y1)(x x1) (x2 x1)(y y1)u003d0
(y2 y1)(x x1) (x2 x1)(y y1)u003d0 pictures 5
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(y2 y1)(x x1) (x2 x1)(y y1)u003d0
(y2 y1)(x x1) (x2 x1)(y y1)u003d0
