Parallel to the existing two oblique lines in the square ABCD, they intersect at point A and point B respectively. Four diagonal lines form an inclined shaft shape and divide the square into nine pieces. Except that the middle one is a small square, and the intersection point is GHIJ, the other eight pieces are four congruent triangles and four congruent trapezoid. Because BE=BF, the small triangle BJE can be cut to the outside of the trapezoid BFGJ to form a square, which is congruent with the square GHIJ, that is, the area before the quadrangle BEGF is cut is the area of the small square GHIJ. Similarly, eight squares outside the cross can be cut and repaired into four squares, so the big square is divided into five small squares with an area of = 20 * 20/5 = 80 cm 2. Just what you want.