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A cone is a three-dimensional geometric shape that tapers smoothly from a flat base (frequently, though not necessarily, circular) to a point called the apex or vertex. More precisely, it is the solid figure bounded by a base in a plane and by a surface (called the lateral surface) formed by the locus of all straight line segments joining the apex to the perimeter of the base. The term "cone" sometimes refers just to the surface of this solid figure, or just to the lateral surface. The axis of a cone is the straight line (if any), passing through the apex, about which the base (and the whole cone) has a circular symmetry. In common usage in elementary geometry, cones are assumed to be right circular, where ''circular'' means that the base is a circle and ''right'' means that the axis passes through the centre of the base at right angles to its plane. Contrasted with right cones are oblique cones, in which the axis does not pass perpendicularly through the centre of the base. In general, however, the base may be any shape and the apex may lie anywhere (though it is usually assumed that the base is bounded and therefore has finite area, and that the apex lies outside the plane of the base). A cone with a polygonal base is called a pyramid.〔(''A Review of Basic Geometry'' )〕 == Other mathematical meanings == In mathematical usage, the word "cone" is used also for an "infinite cone", the union of a set of half-lines that start at a common apex point and go through a base. An infinite cone is not bounded by its base but extends to infinity. A "doubly infinite cone", or "double cone", is the union of a set of straight lines that pass through a common apex point and go through a base, therefore double infinite cones extend symmetrically on both sides of the apex. The boundary of an infinite or doubly infinite cone is a conical surface, and the intersection of a plane with this surface is a conic section. For infinite cones, the word ''axis'' again usually refers to the axis of rotational symmetry (if any). Either half of a double cone on one side of the apex is called a "nappe". Depending on the context, "cone" may also mean specifically a convex cone or a projective cone. 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「cone」の詳細全文を読む スポンサード リンク
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