A circle is a plane figure surrounded by curves. Radius, a line segment with one end in the center and one end on the circle is called radius. In the same circle, there are countless radii, all equal. Diameter, the line segment whose two ends pass through the center of the circle is called diameter. The same circle has countless diameters, all of which are equal.

The diameter of the same circle is twice the radius. A circle is an axisymmetric figure with countless symmetry axes, and the symmetry axis is the diameter. The center of the circle determines the position of the circle, and the radius determines the size of the circle. The diameter is the longest line segment in a circle. The circumference of a semicircle is equal to half the circumference plus the diameter.

The area of a semicircle is half that of a circle. In a plane figure with equal circumference, the area of a circle is the largest, and in a plane figure with equal area, the circumference of a circle is the shortest. A triangle with three vertices on a circle and one side with a diameter must be a right triangle.

Knowledge points of circle:

1. A point on the center of the circle is called the center of the circle, represented by O, a line segment with one end at the center and the other end on the circle is called the radius, represented by R, and a line segment with two ends on the circle and passing through the center is called the diameter, represented by D. ..

2. A circle has countless radii and diameters.

3. The center of the circle determines the position of the circle, and the radius determines the size of the circle.

4. Fold the circle in half, then fold it in half to find the center of the circle.

5. The circle is an axisymmetric figure, and the straight line centered on the diameter is the symmetry axis of the circle. A circle has countless axes of symmetry.

6. The diameter of the same circle is twice as long as the radius, which can be expressed as d = 2r or r = d/2.