Radius of circle: r
Diameter: d
Pi: π (the numerical value is between 3. 14 15926 and 3. 14 15927 ... infinite cyclic decimal), and 3. 14 is usually used as the numerical value of π.
Circular area:? ;
Area of circular ring: s great circle -s small circle =π(R2-r2)(R is the radius of great circle and R is the radius of small circle).
Circumference: or?
The circumference of a semicircle: or?
Extended data:
First, the concept of semicircle:
In mathematics (especially geometry), a semicircle is a one-dimensional trajectory of points forming a semicircle. The radian of a semicircle is always 180 (equivalent to π radian or half a circle). It has only one line of symmetry (reflection symmetry).
Two endpoints of any diameter of a circle divide the circle into two arcs, and each arc is called a semicircle. The semicircle should be separated from the semicircle, because the semicircle is just an arc.
It is a half circle, and the center position of the half circle is the center position of its concentric circle. It has only one diameter, but it has countless radii and an axis of symmetry.
Second, the relevant nature:
The (1) circle is an axisymmetric figure, and its symmetry axis is any straight line passing through the center of the circle. A circle is also a central symmetric figure, and its symmetric center is the center of the circle.
Vertical diameter theorem: the diameter perpendicular to the chord bisects the chord and bisects the two arcs opposite the chord. Inverse theorem: bisecting the diameter of a chord (not the diameter) is perpendicular to the chord and bisecting the two arcs opposite the chord.
⑵ The properties and theorems of central angle and central angle.
(1) In the same circle or the same circle, if one of two central angles, two peripheral angles, two sets of arcs, two chords and the distance between two chords is equal, their corresponding other groups are equal respectively.
(2) In the same circle or equal circle, the circumferential angle of an equal arc is equal to half of the central angle it faces (the circumferential angle and the central angle are on the same side of the chord).
The circumferential angle of the diameter is a right angle. The chord subtended by a 90-degree circle angle is the diameter.
The formula for calculating the central angle is θ = (l/2π r) × 360 =180l/π r = l/r (radian).
That is, the degree of the central angle is equal to the degree of the arc it faces; The angle of a circle is equal to half the angle of the arc it faces.
(3) If the length of an arc is twice that of another arc, then the angle of circumference and center it subtends is also twice that of the other arc.
References:
Baidu encyclopedia-semicircle
Baidu encyclopedia-circle area formula