The answer is as follows:

Draw a picture first and mark the corners as auxiliary lines A'C', DC', BC'. Because the angle AA'C' is a right angle (because this is a cuboid, AA' is perpendicular to the surfACe A'B'C'D, so AA' is perpendicular to A'C'), and the angle A'AC' is 60 degrees, so we can assume that the length of A. hypotenuse AC' is 2H, so we can know that the width of AD is also h, DC'= root sign 3H. Therefore, we can know that the rectangle AA'D'D is a square with a side length of H, so BB'C'C is also a square with a side length of H. In a right triangle A'B'C', A'C'= root 3H, B'C'=H, then A'B'= root 2H, then the same AB = A. Note: the root number can't be typed, so it is replaced by a word. )