Chengjiang bazi grass-planting brick

Three small triangles can make a square; Because the external angle of the regular octagon is 45, 45+45 equals 90; The sides of a regular octagon are equal, so the completed quadrilateral is a diamond, and the angle of 90 is a square, so the area of the completed square is the square of A, and the area of the middle square is the square of 2a.