The set of points whose distance from a point on a plane is equal to a fixed length is called a circle, with the fixed point as the center and the fixed length as the radius of the circle. The standard equation of a circle: (x-a)? +(y-b)？ =r？ . (a, b) is the coordinate of the center of the circle, and the general equation of the circle: x? +y？ +Dx+Ey+F=0 .

A circle is a geometric figure. By definition, compasses are usually used to draw circles. The diameter and radius of the inner circle of the same circle are always the same, and the circle has countless radii and diameters. A circle is a figure with axial symmetry and central symmetry. The axis of symmetry is the straight line where the diameter lies. At the same time, the circle is a "positive infinite polygon" and "infinity" is just a concept. A circle can be regarded as a regular polygon composed of countless infinitesimal points. The more sides a polygon has, the closer its shape, perimeter and area are to a circle. So there is no real circle in the world, and the circle is actually just a conceptual figure.