1, diameter = radius ×2 formula: d=2r? .

2. Radius = diameter ÷2 Formula: r=d÷2? .

3. circumference = π× diameter formula: c=πd=2πr? .

4. Area of circle = radius × radius× π formula: S = π rr.

5. Semi-circular perimeter formula: C=πd÷2+d or C=πr+2r.

Knowledge about circles

1. circle: 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.

2. Center of the circle: the intersection of any two symmetrical axes of a circle is the center of the circle, which is generally represented by the symbol O. ..

3. Diameter: The line segment whose two ends pass through the center of the circle is called the diameter of the circle. The diameter is usually indicated by the letter d.

4. Radius: The line segment connecting the center of the circle and any point on the circle is called the radius of the circle. Radius is generally represented by the letter R.

A circle has countless diameters and radii. The circle is an axisymmetric figure, and the straight lines of each diameter are the symmetry axis of the circle.