Proof of the formula of circular area. In chapter 9, the formula of circular area S= 12Lr is proposed, where S, L and R are circular area, perimeter and radius respectively. Liu Hui proved it with the thought of limit. Finally, the regular polygon combined with the circumference is divided into countless isosceles triangles with the center of the circle as the vertex and the side length as the bottom. Since the radius multiplied by the seaside is equal to twice the area of each small triangle, the sum of the areas of countless small triangles should be the product of the radius and the semicircle, as Liu Hui said: "The radius multiplied by an edge and cut it, every work is self-doubling, then the radius multiplied by the semicircle is the power of the circle."