In ancient times, this problem was almost entirely induced by experiments. It is found in experience that there is a constant ratio between the circumference and diameter of a circle, which is called pi (recorded in the west). So naturally, the circumference of a circle is: or (where is the diameter and radius of the circle).

Later mathematicians tried to work out the specific value of π. Mathematician Liu Hui used the secant method, that is, the circumference of a circle inscribed with a regular polygon and circumscribed with a regular polygon is close to the circumference of a circle, and the circle is close to a 192 polygon with a pi of about 3. 14.

The general method of cutting circle is found in middle school mathematics textbooks. But it must be noted that, to a great extent, it is only a method to calculate pi, and it seems to be a fact that pi is C = π * d, and this method is only to determine the value of π. If you think about it carefully, you will know that this is problematic, because they have not logically proved that the circumference of a circle is really proportional to the diameter. Furthermore, their concept of circumference is only intuitive and irrational.