Story: In 263 AD, Liu Hui annotated Nine Chapters of Arithmetic. He found that "the diameter is three times that of a week" is just the ratio of the circumference to the diameter of a regular hexagon inscribed in a circle. He founded secant technology, and thought that when the number of inscribed sides of a circle increased infinitely, the circumference was closer to the circumference of a circle.

He calculated the pi of a regular 3072 polygon inscribed in a circle = 3927/1250. Liu Hui applied the concept of limit to solving practical mathematical problems, which is also a great achievement in the history of mathematics in the world.

The nature of the circle:

1. In the same circle or the same circle, if the distance between two central angles, two peripheral angles, two groups of arcs and one center of two chords is equal, the corresponding other groups are equal.

2. 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.

3. Triangle has definite circumscribed circle and inscribed circle. The center of the circumscribed circle is the intersection of the perpendicular lines of each side of the triangle, and the distances to the three vertices of the triangle are equal.

4. If two circles intersect, the line segment connecting the centers of the two circles (or a straight line can be used) bisects the common chord vertically.