"Circle" in mathematics [Definition of Circle] Geometry says: A figure composed of all points whose distance from a plane to a fixed point is equal to a fixed length is called a circle. A fixed point is called the center of the circle and a fixed length is called the radius. Trajectory theory: the trajectory of a moving point on a plane with a certain length as the center is called a circle. Set theory: The set of points whose distance to a fixed point is equal to a fixed length is called a circle. [Relative quantity of a circle] Pi: The ratio of the circumference to the diameter and length of a circle is called Pi, and its value is 3.14159265358979323846 …, which is usually expressed by π. In calculation, 3. 14 16 is often taken as its approximate value. Arc chord: the part between any two points on the circle is called arc, or simply arc. An arc larger than a semicircle is called an upper arc, and an arc smaller than a semicircle is called a lower arc. A line segment connecting any two points on a circle is called a chord. The chord passing through the center of the circle is called the diameter. Central angle and central angle: the angle of the vertex on the center of the circle is called the central angle. The angle at which the vertex is on the circumference and both sides intersect with the circle is called the circumferential angle. Inner and outer center: the circle passing through the three vertices of the triangle is called the circumscribed circle of the triangle, and its center is called the outer center of the triangle. A circle tangent to all three sides of a triangle is called the inscribed circle of the triangle, and its center is called the heart. Sector: On a circle, the figure enclosed by two radii and an arc is called a sector. The development diagram of the cone is a sector. The radius of this sector becomes the generatrix of the cone. [Letter representation of the correlation between circles] Circle-⊙ Radius -R arc-⌒ Diameter -D fan arc length/conic generatrix -L circumference -C area-S [positional relationship between circle and other figures] The positional relationship between circle and point: Take point P and circle O as examples (let p be a point, then PO is the distance from the point to the center), and p is in [ P on ⊙O，po = r； P is within ⊙O, and PO r；; AB is tangent to ⊙O, po = r；; AB intersects with ⊙O, and PO < R. There are five positional relationships between two circles: if there is nothing in common, one circle is called external separation outside the other circle, and it contains; If there is only one common point, a circle is called circumscribed by another circle and inscribed by another circle; There are two things in common called intersection. The distance between the centers of two circles is called the center distance. The radii of the two circles are R and R respectively, and R≥r, and the center distance is P: outward separation P > R+R; Circumscribed p = r+r; Intersection r-r < p < r+r; Inner cut p = r-r; Contains p; 0, a circle and a straight line have two intersections, that is, a circle and a straight line intersect. If b 2-4ac = 0, the circle and the straight line have 1 intersections, that is, the circle is tangent to the straight line. If b 2-4ac