A (1) circle has countless radii and diameters, and all the diameters and radii are equal in the same circle and the same circle.

(2) The center of the circle is represented by O, the radius by R and the diameter by D..

(3) A circle has numerous symmetry axes.

(4)d=2r

(5) the circumference of a circle: d=C

(6) Draw the largest circle in the square with a ratio of 3. 14/4.

(7) Draw the largest' square' in the circle with the ratio of 2/3. 14.

(8) Pi is a fixed value, usually 3. 14, with? express

(9) The formula of circular area =r? r

(10) Ring area = large circle area-small circle area.

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