Area is a measure of space occupied in 2 dimensions. The area of the considered two dimensional cross section is:
When some high dimensional object is sliced by hyperplane, then the area of the intersection of the hyperplane with the object is the cross-section.
For the given case, the right rectangular pyramid sliced will be having a triangle with height same as that of the pyramid but base equal to either length or the width of the base rectangle, or the diagonal.
Thus, if we consider: L = length of base, W = width of base , D = diagonal's length of the base of the rectangular pyramid, with the height h units, then we get the cross section's area as:
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