The surface area of this rectangular box without cover is:
S rectangular ABFE+S rectangular BCGF+S rectangular CDHG+S rectangular ADHE+S rectangular ABCD,
Also, the length of this cuboid is x cm, the width is y cm and the height is z cm.
∴S rectangular ABFE+S rectangular BCGF+S rectangular CDHG+S rectangular ADHE+S rectangular ABCD=xz+yz+xz+yz+xy=2xz+2yz+xy (square centimeter)
The paper area required by Xiaogang (excluding the joint paper) is at least 2xz+2yz+xy square centimeter.
So the answer is: 2xz+2yz+xy.