Because the inscribed circle area of a circle is 1 of its circumscribed circle area, the formula of China side area of a circle is; The circular area is 7a, and its inscribed square area is 4.5a (note that a= 1 one third of the diameter d;

Is the area between a square and a circle (square and inscribed circle) 9(d/3)? -7(d/3)？ =2(d/3)？

The area between a circle and a square (circle and inscribed square) is: 7(d/3)? -4.5(d/3)？ =2.5(d/3)？

Is the area between a square and a square (circumscribed circle and inscribed circle of a circle) 9(d/3)? -4.5(d/3)？ =4.5(d/3)？

The nature of the circle:

1, the circle is an axisymmetric figure, and its symmetry axis is an arbitrary straight line passing through the center of the circle. A circle is also a central symmetric figure, and its symmetric center is the center of the circle.

2. If the center distances of two central angles, two external angles, two groups of arcs, two chords and one group of chords are equal in the same circle or an equal circle, then their corresponding other groups are equal respectively.

3. In the same circle or equal circle, the circumferential angle of an equal arc is equal to half of the central angle it faces (the circumferential angle and the central angle are on the same side of the chord).

A triangle has a unique 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; The center of the inscribed circle is the intersection of the bisectors of the inner angles of the triangle, and the distances to the three sides of the triangle are equal.