About the Book
Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Pages: 66. Chapters: Constellations listed by Ptolemy, Works by Ptolemy of Alexandria, Libra, Capricornus, Ursa Major, Aries, Scorpius, Virgo, Sagittarius, Pisces, Ursa Minor, Taurus, Leo, Cancer, Aquarius, Ophiuchus, Canis Minor, Lupus, Argo Navis, Cygnus, Canis Major, Centaurus, Lyra, Piscis Austrinus, Cetus, Triangulum, Corona Borealis, Delphinus, Equuleus, Lepus, Bootes, Serpens, Corona Australis, Orion, Ara, Andromeda, Almagest, Pegasus, Ptolemy's theorem, Geography, Cassiopeia, Gemini, Hercules, Aquila, Auriga, Eridanus, Perseus, Draco, Corvus, Cepheus, Hydra, Crater, Canon of Kings, Ptolemaeus, Ptolemy's world map, Agathodaemon, Planisphaerium, Agathedaemon of Alexandria. Excerpt: In Euclidean geometry, Ptolemy's theorem is a relation between the four sides and two diagonals of a cyclic quadrilateral (a quadrilateral whose vertices lie on a common circle). The theorem is named after the Greek astronomer and mathematician Ptolemy (Claudius Ptolemaeus). Ptolemy used the theorem as an aid to creating his table of chords, a trigonometric table that he applied to astronomy. If the quadrilateral is given with its four vertices A, B, C, and D in order, then the theorem states that: where the vertical lines denote the lengths of the line segments between the named vertices. This relation may be verbally expressed as follows: If a quadrilateral is inscribable in a circle then the product of the measures of its diagonals is equal to the sum of the products of the measures of the pairs of opposite sides.Moreover, the converse of Ptolemy's theorem is also true: In a quadrilateral, if the sum of the products of its two pairs of opposite sides is equal to the product of its diagonals, then the quadrilateral can be inscribed in a circle. Equilateral trianglePtolemy's Theorem yields as a corollary a pretty theorem regarding an equilateral triang...
