Plane and solid geometry . BOOK VII 351 749. Defs. A pyramid is triangular, quadrangular, etc., ac-cording as its base is a triangle, a quadrilateral, etc. 750. Questions. How many faces has a triangular pyramid ? a tet-rahedron ? Can these terms be used interchangeably ? How many dif-ferent bases may a triangular pyramid have ? 751. Def. The altitude of a pyramid is the perpendicularfrom the vertex to the plane of the base, as VO in the figurebelow, and in the figure on preceding RegularPyramid Truncated Frustum of Frustum of Pyramid Triangular Pyramid Regular Pyramid 752. Def. A regul
Plane and solid geometry . BOOK VII 351 749. Defs. A pyramid is triangular, quadrangular, etc., ac-cording as its base is a triangle, a quadrilateral, etc. 750. Questions. How many faces has a triangular pyramid ? a tet-rahedron ? Can these terms be used interchangeably ? How many dif-ferent bases may a triangular pyramid have ? 751. Def. The altitude of a pyramid is the perpendicularfrom the vertex to the plane of the base, as VO in the figurebelow, and in the figure on preceding RegularPyramid Truncated Frustum of Frustum of Pyramid Triangular Pyramid Regular Pyramid 752. Def. A regular pyramid is a pyramid whose base is aregular polygon, and whose vertex lies in the perpendicularerected to the base at its center. 753. Def. A truncated pyramid is the portion of a pyramidincluded between the base and a section of the pyramid madeby a plane cutting all the edges. 754. Def. A frustum of a pyramid is the portion of a pyra-mid included between the base and a section of the pyramidmade by a plane parallel to the base. 755. The following are some of the properties of a pyramid;the student should prove the correctness of each: (a) TJte lateral edges of a regular jjyramid are equal, (b) The lateral edges of a frustum of a regidar pyramid are equal. (c) The lateral faces of a regular pyramid are equal isoscelestriangles. (d) TJie lateral faces of a frustum of a regular pyramid areequal isosceles trapezoids. 352 SOLID GEOMETRY 756 I. 11. Propositiox II. Theorem ? If
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