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In geometry a conoid (Greek: κωνος cone and -ειδης similar) is a ruled surface, whose rulings (lines) fulfill the additional conditions :(1) All rulings are parallel to a plane, the ''directrix plane''. :(2) All rulings intersect a fixed line, the ''axis''. *The conoid is a right conoid, if its axis is perpendicular to its directrix plane. Hence all rulings are perpendicular to the axis. Because of (1) any conoid is a Catalan surface and can be represented parametrically by * Any curve with fixed parameter is a ruling, describes the ''directrix'' and the vectors are all parallel to the directrix plane. The planarity of the vectors can be represented by :. *If the directrix is a circle the conoid is called circular conoid. The term ''conoid'' was already used by Archimedes in his treatise ''On conoids and spheroides''. == 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「conoid」の詳細全文を読む スポンサード リンク
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