The side length of a square is a, and the intersection of diagonals is o. The volume v of triangular pyramid ABCD can be obtained from the bottom area ACD and the height BO 1 (assuming the height is diagonal BD). Let the included angle BOD between the surface ABC and the bottom ACD be x, then

V =112* square root 2 * primary cube *sinθ,

In this way, when the maximum volume is obtained, sinθ= 1, which is 90.

At this time, it happens that the plane ABC is perpendicular to the bottom surface ACD, and there is a triangular relationship △BOD is isosceles Rt△, so it is easy to get that the angle ∠DBO formed by the straight line BD and the plane ABC is 45.