"Decagon" Quotes from Famous Books
 ... by an angle of 2 α {a} (crd. 2 α {a})/120 sin α {a}. Ptolemy first gives the minimum number of geometrical propositions required for the calculation of the chords. The first of these finds (crd. 36°) and (crd. 72°) from the geometry of the inscribed pentagon and decagon; the second ('Ptolemy's Theorem' about a quadrilateral in a circle) is equivalent to the formula for sin (θ-φ) {th-ph}, the third to that for sin ½ θ {th}. From (crd. 72°) and (crd. 60°) Ptolemy, by using these propositions ...
— The Legacy of Greece • Various |
