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Frustum of a pyramid

Drawing of a frustum pyramid
It is possible to calculate the side length a of a frustum, if the height h and the lengths of the perpendicular bisectors m1 and m2 are given. The perpendicular bisectors are the lines from the bases center points to the edge. (se the figure to the right).

As you can see on the figure, the dotted lines are forming a right triangle; hence it’s possible to use the following version of the Pythagorean theorem:
a=sqrt{h^2+(m_1-m_2)}