2. The typical abbreviation of radius and the name of mathematical variable are called R. By extension, the diameter d is defined as twice the radius: d=2r.
Extended data:
I. Diameter characteristics
A circle can have countless diameters (referring to the line segment itself), but any point on the plane except the center of the circle has only one diameter. One end of the diameter is called the diameter point at the other end. Every point on the circumference has one and only one diameter point.
Diameter divides the circle into two parts with equal area (each part becomes a semicircle) and divides the circumference into two parts with equal length. The midpoint of the diameter is the center of the circle, and the diameter is also the longest chord on the circle. In other words, the diameter of a circle is the maximum distance between any two points on the circumference. In the same circle, the diameter is equal to twice the radius (r). The ratio of the circumference to the diameter of a circle is π.
Given a circle and a diameter AB (A on the circle (A and B are points on the circle)), the angle ACB is the right angle of any other point C on the circle. If point C is outside the circle, the ACB angle is acute, and if point C is inside the circle, the ACB angle is obtuse.
Second, the ruler drawing
In ruler drawing, if a circle and its center are known, just draw a straight line through the center, and the line segment between the intersection of this straight line and the circle is the diameter of the circle. If the center of the circle is unknown, the diameter can be made as the middle perpendicular of the chord. The specific method is: take any chord of a circle as the middle vertical line of this chord, then the line segment between the middle vertical line and the two intersections of the circle is the diameter of the circle.
If you want to find the diameter of a fixed point on a circle without knowing the center of the circle, you can use the characteristic that a point on the circle makes an angle of 90 degrees with the diameter: first, you can make a chord at a given point and intersect with another point. Then cross another point to make a straight line perpendicular to the chord, intersect at the third point, and connect the original given point with the third point, which is the required diameter.
Third, the diameter of the ball.
For spheres in three-dimensional space, you can also define the diameter and radius. The diameter of the sphere is the diameter of any great circle (the circle obtained by cutting the sphere through the center plane of the sphere). The diameter of a ball, like the diameter of a circle, is the maximum distance between two points on the ball, and only one diameter can be made through each point on the ball.