In the same plane, the distance from the known point O to the straight line L is 5. Draw a circle with the center of the circle and the radius of R. when r =? There are only three points on the circle O whose distance to the straight line L is equal to 3?

Make the parallel line of L, so that the distance from L to L is 3. Note that the above is L1, and the following is L2. The problem is transformed into "when R is, there are only three intersections between the circle and L1, L2, obviously r = 8

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