When the area of quadrilateral ABCD is larger than that of quadrilateral BCC 1B 1, that is, when two quadrangular prisms are bottom ABCD, the overlapping surface area is the smallest, that is, 18k 2 >: 5k, so k & gt5/ 18;

Conversely, when the area of quadrilateral ABCD is smaller than quadrilateral BCC 1B 1, the new quadrangular prism should be a cuboid, that is, 18k 2.

But I want to ask why there are four kinds, and I just want to come up with three schemes.