|
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).〔C. Ptolemy, ''Almagest'', Book 1, Chapter 10.〕 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. In the context of geometry, the above equality is often simply written as : AC·BD=AB·CD+BC·AD. 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.'' ==Examples== 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ptolemy's theorem」の詳細全文を読む スポンサード リンク
|