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How to judge whether the slope of a straight line tangent to a circle exists in the second compulsory course of senior high school mathematics?
In fact, it is to judge whether the point is in the circle, and there is no slope outside the circle and on the circle.
That is (x 1-x)? +(y 1-y)？ ≥R？ Slope existence
(x 1, y 1) point coordinates (x, y) point coordinates r the radius of the circle.