The concept of set form:

1, a circle can be regarded as a set of points whose distance to a fixed point is equal to a fixed length;

2. Outside the circle: it can be considered as a set of points whose distance to a fixed point is greater than a fixed length;

3. The interior of a circle can be regarded as a collection of points whose distance to a fixed point is less than a fixed length.

The concept of trajectory form:

1. The distance to a fixed point is equal to the trajectory of a fixed-length point, and the trajectory is a circle with the fixed point as the center and the fixed length as the radius.

Second, the positional relationship between a point and a circle

1, the point in circle d

2. The point is on the circle d=r, and the point B is on the circle;

3, the point is outside the circle d>r point A is outside the circle.

Third, the positional relationship between a straight line and a circle

1, separation of straight line and circle d >；; R has no intersection;

2. The straight line intersects with the tangent of circle D = r;

3. the intersection of a straight line and a circle D < r has two intersections.