Calculation formula of circle
1, the circumference of the circle c = 2π r = π d.
2, the area of the circle S=πr?
3. Sector arc length l=nπr/ 180
4. Sector area S=nπr? ，360=rl/2
5, cone transverse area S=πrl
Equation of circle:
The standard equation of 1. circle: In the plane rectangular coordinate system, the standard equation of a circle with a radius of r and a center of point O(a, b) is (x-a) 2+(y-b) 2 = r 2.
2. General equation of the circle: expand the standard equation of the circle, move the term and merge the similar terms, and the general equation of the circle can be obtained as X 2+Y 2+DX+EY+F = 0. Compared with the standard equation, in fact, D=-2a, E=-2b and f = a 2+b 2.
Theorem of circle
1. Central Angle Theorem: In the same circle or in the same circle, an equal central angle has equal arc, chord and chord center distance.
Inference: In the same circle or equal circle, if one set of quantities in two central angles, two arcs, two chords or the distance between two chords is equal, the corresponding other set of quantities is also equal.
2. Theorem of circumference angle: The circumference angle of an arc is equal to half of the central angle it faces.
Inference 1: the circumferential angles of the same arc or equal arc are equal; In the same circle or in the same circle, the arcs corresponding to equal circumferential angles are also equal.
2: The circumferential angle of a semicircle (or diameter) is a right angle; Infer from the angle of a 90-degree circle.
3: If the median line of one side of a triangle is equal to half of this side, then this triangle is a right triangle.