Quick overview:
  • What is a circle?
  • Formula for calculating the circumference of a circle when the radius is known.
  • Formula for calculating the circumference of a circle when the diameter is known.
  • What value of pi can be used to calculate the perimeter?
  • Why are the letters C, S, r, and d used to represent the measurements of a circle?

What is a circle?

A circle is the set of all points lying on a plane and equidistant from a given point (called the center) by a constant distance (called the radius – r). The center of the circle is the fixed point in the middle. Diameter (d): the line segment passing through the center and connecting two points on the circle, equal to d=2r.

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Formula for calculating the circumference of a circle when the radius is known.

According to the textbook "Connecting Knowledge with Life," Math 5, Volume 1, Lesson 27, to calculate the circumference of a circle, we multiply 3.14 by 2 and then multiply by the radius.

C = π x 2 xr

In there:

C is the circumference of the circle.

d is the diameter of the circle.

r is the radius of the circle.

π is a constant (equivalent to 3.14)

Example: Calculate the circumference of a circle with a radius of 5cm.

Answer: Circumference of circle C = 3.14 x 2 x r = 3.14 x 2 x 5 = 31.4 (cm)

Formula for calculating the circumference of a circle when the diameter is known.

Since the diameter is twice the radius, to calculate the circumference of a circle given the diameter, we multiply 3.14 by 2 and then multiply by the radius.

C = π xd

Example: Calculate the circumference of a circle with a diameter of 6 dm.

Answer: Circumference of circle C = 3.14 x d = 3.14 x 6 = 18.84 (dm)

What value of pi can be used to calculate the perimeter?

According to the textbook "Connecting Knowledge with Life," in the section on rounding decimal numbers, the number π\pi is introduced as a special number with many digits after the decimal point: "3.141592..." and requires rounding to the tenth and hundredth places.

According to the Grade 6 Math textbook (Workbook – new curriculum), the textbook explains that π/π is Archimedes' constant, equal to the ratio between the circumference of a circle and its diameter, with an approximate value of "3.141592653589793..." and includes exercises on rounding this number.

Therefore, depending on the lesson requirements, an appropriate rounded value of pi can be chosen.

Why are the letters C, S, r, and d used to represent the measurements of a circle?

C stands for "Circumference", S stands for "Surface Area" (or simply "area"), r stands for "Radius", and d stands for "Diameter".

The use of these characters makes mathematical notation concise, easy to remember, and widely used throughout the world .

What is the formula for calculating the area of ​​a circle? The basic formula for calculating the area of ​​a circle is S = r x r x π, where S is the area, r is the radius, and π is the constant (approximately 3.14159). Alternatively, if the circumference and diameter are known, the area of ​​a circle can also be calculated.

Source: https://vietnamnet.vn/cong-thuc-tinh-chu-vi-hinh-tron-2445269.html