. 51. OF SIMPLE FORMS IN PARTICULAR. 



14. Let a. be the plane angle at the apex ; we have 

 2 a 2 9 

 2(a a + 9> 



cos. a. 



. 51. PYRAMIDS IN GENERAL. 



A pyramid is contained under equal and simi- 

 lar triangles. 



1. The whole number of these triangles, as well as its 

 half, is an even number. 



2. The triangles are either isosceles or scalene. A pyra- 

 mid contained under isosceles triangles, is termed an Isosce- 

 les pyramid; one contained under scalene triangles, a scalene. 

 pyramid. 



3. The angles at the vertex of these triangles, are the 

 Apices of the pyramid. 



4. The straight line through the apices is the Principal 

 Axis, 



5. The edges contiguous to the terminal points of the 

 axis, are called Terminal Edges ; they are equal in isosceles, 

 and unequal in scalene pyramids. The remaining edges of 

 the pyramid either lie in a plane perpendicular to the prin- 

 cipal axis, or they are situated like the lateral edges of a 

 rhombohedron (. 50. 6.). They are called Lateral Edges, 

 the first of them sometimes Edges at the Base. 



6. The pyramids are divided according to the whole, 

 and denominated according to half the number of their faces, 

 as follows : 



