Mathematically, the formula for the area of a circle is πr2, where π is a constant, which is approximately equal to 3. 14, and r is the radius of the circle. When drawing a square in a circle, the diagonal of the square is the diameter of the circle, and the side length of the square is the radius of the circle. So the area of the square is r2.

The square area is part of the circular area, so you need to subtract the square area from the circular area to get the shadow area. This difference is 0.86r2, so the formula of the area of the outer and inner circle is 0.86r2, which is calculated according to the area of the circle and the area of the square.