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A simple proof of the Beijing theorem
Published online by Cambridge University Press: 01 August 2016
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If a square pyramid is cut obliquely by a plane as shown in figure 1, a pentagonal cross-section PQSTR is generated. The “Beijing theorem” states that the only case in which this pentagon can be regular is that in which the pyramid is half an octahedron and (except for a complication to be discussed later) the coordinates of the points P, Q, R, S and T are uniquely determined.
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- Copyright © The Mathematical Association 1992
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