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若直线l与圆锥曲线相交于点A(x1,y1),B(x2,y2)时,则弦AB的长:由和即可导出这个公式.
If the line l and the conic curve intersect at the point A (x1, y1), B (x2, y2), then the length of the string AB: From and can be derived from this formula.