118
Triangles and pyramids
Textbook: Part 4, lesson 118, pp. 31–33
GoalTriangles and pyramids: the base, side faces and apex of a pyramid; triangular and square pyramids.
New words
pyramid · piramidaapex · piramida uchibase · asosside face · yon yoq
Explanation
A pyramid has one base, which can be any polygon. Its side faces are triangles, and they all meet at one point, the apex. A pyramid with a triangular base is a triangular pyramid; with a square base, a square pyramid. A triangular pyramid has 4 faces, 6 edges and 4 vertices. A square pyramid has 5 faces, 8 edges and 5 vertices.
Worked examples
The Egyptian pyramids are square pyramids: a square base and 4 triangular side faces.
In a triangular pyramid all faces are triangles: 4 of them.
Class activity
“Pyramid model”: build triangular and square pyramids from plasticine balls and sticks and count the vertices and edges.
Practice
1
How many faces does a triangular pyramid have?
4
2
How many edges does a triangular pyramid have?
6
3
How many edges does a square pyramid have?
8
4
What shape and how many are the side faces of a square pyramid?
triangles, 4
5
What is the sum of the numbers of faces and vertices of a triangular pyramid?
8