2. Fold a circular piece of paper twice, and the point where the crease intersects the center of the circle is called the center of the circle. The center of the circle is generally represented by the letter O, and its distance to any point on the circle is equal.

3. Radius: The line segment connecting the center of the circle and any point on the circle is called radius. The radius is generally represented by the letter R. If the two feet of a compass are separated, the distance between the two feet is the radius of the circle.

4. The center of the circle determines the position of the circle and the radius determines the size of the circle.

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

6. In the same circle, all radii are equal and all diameters are equal.

7. The same circle has countless radii and countless diameters.

8. The diameter of the same circle is twice the radius, and the radius is half the diameter.

Represented by letters: D = 2RR = D

Expressed in words: radius = diameter ÷2 diameter = radius ×2.

9. Circumference: The length of the curve around a circle is called circumference.

10. The circumference of a circle is always greater than 3 times the diameter, and this ratio is a fixed number. We call the ratio of the circumference to the diameter of a circle pi, which is expressed by letters. Pi is an infinite cyclic decimal. In the calculation, take 3. 14. The first person in the world to calculate pi was China mathematician Zu Chongzhi.

1 1. The circumference formula of a circle: C=d or C=2r.

Perimeter = × Diameter Perimeter = × Radius ×2

12. Area of the circle: The area occupied by the circle is called the area of the circle.

13. Cut a circle into an approximate rectangle. The length of the cut rectangle is equal to half the circumference of the circle, which is indicated by the letter (R), and the width is equal to the radius of the circle, which is indicated by the letter (R). Because the area of a rectangle = length× width, and the area of a circle = r×r R. The area formula of a circle: S = R2.

14. The area formula of the circle: S = R2 or S=(d2)2 or S=(C 2)2.

15. Draw the largest circle in a square, and the diameter of the circle is equal to the side length of the square.

16. Draw the largest circle in the rectangle, and the diameter of the circle is equal to the width of the rectangle.

17. A ring, the radius of the outer circle is R, the radius of the inner circle is R, and its area is S = R2-R2 or S = (R2-R2).

(where r = the width of the ring. )

19. The circumference of a semicircle is equal to half the circumference plus the diameter. The difference between the circumference of a semicircle and that of a semicircle is that a semicircle has a diameter and a semicircle has no diameter.

The perimeter formula of a semicircle: C = D2+D or C = R+2R.

Half of the circumference =r