Euclid:Proposition/XIV/4

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Proposition

In the words of Hypsicles of Alexandria:

If $ABC$ be an equilateral triangle in a circle, $D$ the centre, and $DE$ perpendicular to $BC$,
$30 BC . DE =$ (surface of icosahedron).

(The Elements: Book $\text{XIV}$: Proposition $4$)


Sources