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我们知道直线与二次曲线相切的问题常可用判别式法(以△=0为其特征)来解,从运动的观点来看,△>0到△=0的几何表现是直线与二次曲线从有两个公共点到逐渐重合为
We know that the problem of tangent between a straight line and a quadratic curve can often be solved by the discriminant method (with △=0 as its characteristic). From the point of view of motion, the geometric performance of △>0 to △=0 is linear and quadratic. The curve goes from two common points to gradually overlapping