2. Center: The intersection of any two symmetrical axes of a circle is the center. Note: the center of the circle 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.
6. In the same or equal circle: the diameter is twice the radius, and the radius is half the diameter. D=2r or r=d/2.
7. The radius or diameter of a circle determines the size of the circle, and the center of the circle determines the position of the circle.
8. Circumference of the circle: The length of the curve around the circle is called the circumference of the circle, which is represented by the letter C.
9. Pi: The ratio of the circumference to the diameter of a circle is called pi.
10, the quotient of the circumference divided by the diameter of the circle is a fixed number, called pi. It is an infinite cycle decimal (irrational number), represented by the letter π. When calculating, we usually take its approximate value, π≈3. 14.
1 1, and the circumferential angle of the diameter is a right angle. A chord with a circumferential angle of 90 is a diameter.
12, area formula of the circle: the size of the plane occupied by the circle is called the area of the circle. πr^2； , represented by the letter s.
13. In the same circle or circle, the isocentric angle has equal arc, chord and chord center distance.
14. If two arcs are equal in the same circle or equal circle, then their central angles are equal, their chords are equal and their chords are equidistant.
Second, the perimeter calculation formula
(1) Known diameter: C=πd
(2) Known radius: C=2πr
(3) Known perimeter: D=c/π
(4) Half circumference: 1/2 circumference (curve)
(5) The circumference of a semicircle: 1/2 circumference+diameter (π÷2+ 1)
Three, the area calculation formula:
(1) Known radius: S=πr2.
(2) Known diameter: S=π(d/2)2
(3) Known circumference: S =π[c \u( 2π)]2