Radius: In mathematical geometry, it means half the distance between the longest two points on a circle. It is called radius, and the diameter is twice the radius, which is equivalent to the radius multiplied by 2 equals the diameter.
Diameter: refers to the distance from the center of a plane figure or solid (such as a circle, a conic curve, a sphere or a cube) to two points on the side, usually represented by the letter "D".
Circumference of a circle: A regular N-polygon is connected in the circle, the side length is set to an and the circumference of the regular N-polygon is n*an. When n increases, the perimeter of a regular polygon is close to the perimeter of a circle. Calculation: Experience shows that there is a constant ratio between the circumference and diameter of a circle, and this constant is called pi. Then naturally, the circumference of a circle is: C = π * d or C=2*π*r (where d is the diameter of the circle and r is the radius of the circle).
Pi: expressed by π, mathematician Liu Hui uses the secant method, that is, the circumference of a circle inscribed with a regular polygon and circumscribed with a regular polygon is close to that of a circle, and the circle is close to 192 polygons, and the pi is about 3. 14.