Pyramid - Pyramid

If there is a polygon P on a plane α and a point O outside α, the solid formed by all the line segments connecting any point on O and P is called a pyramid, with O being the vertex, P being the base, and the faces that are not the base being the sides. The distance from O to α is called the height. The volume of a pyramid with a base area S and height h is V = (1/3)Sh. Pyramids are called triangular pyramids, square pyramids, or polygonal pyramids depending on whether the base is a triangle, square, or polygon. The cut surface of a polygonal pyramid cut by a plane parallel to the base is a polygon similar to the base, and the part between this and the base is called a truncated pyramid. When the base is a regular polygon and the line connecting its center and the vertex is perpendicular to the base, the pyramid is called a regular pyramid. The sides of a regular pyramid are made up of congruent isosceles triangles.

[Minoru Kurita]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

平面α上に多角形P、αの外に点Oがあるとき、OとPの任意の点を結んでできる線分全体のつくる立体を角錐といい、Oを頂点、Pを底面、底面でない面を側面という。また、Oからαに至る距離を高さという。底面の面積がS、高さhの角錐の体積はV＝(1/3)Shである。角錐は、底面が三角形、四角形、多角形であるのに応じて、三角錐、四角錐、多角錐という。多角錐を、底面に平行な平面で切った切り口は、底面に相似な多角形であり、これと底面の間にある部分を角錐台という。底面が正多角形で、その中心と頂点を結ぶ直線が底面に垂直のとき、この角錐を正角錐という。正角錐の側面は合同な二等辺三角形からできている。

［栗田　稔］

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例