OA=OB The radius of two small circles is R 1=R2. Compared with two points, one is point O and the other is point C. Connect point C to the center of the two small circles to get a quadrilateral. Because its four sides are equal, the quadrilateral is a rhombus with equal diagonals, so it is a square.

Shadow area =2× (the area of a quarter circle-the area of an isosceles right triangle with the radius of a small circle as its side) = the area of a semicircle-the area of a square with R 1 as its side (R2).