1. Circumference formula of the circle: C= πd or c = 2 π r.

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

3. Cut a circle into an approximate rectangle. The length of the cut rectangle is equivalent to half of the circumference, and the width is equivalent to the radius of the circle. Because the area of a rectangle = length × width, and the area of a circle = π× r× r.

4. Formula of circular area: s = π r? Or S= π( d\2)? Or S= π(C÷π÷2)?

Draw the largest circle in a square. The diameter of the circle is equal to the side length of the square.

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

7. A circle, the radius of the outer circle is r, the radius of the inner circle is r, and its area is S=πR? -πr？ Or S=π(R? -r？ )。 (where r = the width of the ring. )

8. Ring circumference = outer circumference+inner circumference

9. The circumference of a semicircle is equal to half the circumference plus the diameter. The formula of the circumference of a semicircle is: c = π d ÷2+d or c = π r+2R 10. The formula of semicircle area = circle area ÷ 2 is: s = π r? ÷ 2

1 1. How many times the radius of the same circle is enlarged or reduced, the diameter and circumference are also enlarged or reduced by the same multiple. And the area is expanded or reduced by the square of the multiple. For example, the radius, diameter and circumference of the same circle are enlarged by four times, and the area is enlarged by 16 times.

12. The radius ratio of two circles is equal to the diameter ratio and the perimeter ratio, and the area ratio is equal to the square of the above ratio. For example, if the radius ratio of two circles is 2: 3, then the diameter ratio and perimeter ratio of these two circles are both 2: 3, while the area ratio is 4: 9.

13. The radius of the circle is increased by one centimeter, and the circumference is increased by 2π one centimeter; When the diameter of a circle increases by one centimeter, the circumference increases by one centimeter.

14. When the perimeters of rectangle, square and circle are equal, the area of circle is the largest and the area of rectangle is the smallest.