Proposition·Untested·2605.00009

Proposition XIII.13

To construct a pyramid (regular tetrahedron), to comprehend it in a given sphere, and to prove that the square on the diameter of the sphere is one and a half times the square on the side of the pyramid.

Proof

Inscribe an equilateral triangle (IV.2); erect an apex above the centroid at height where is the circumradius. The four equal edges form the tetrahedron; place the sphere through its four vertices. The diameter-squared side-squared .

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