Class 8 Chapter 13: Representing Solids on a Paper

# Class 8 Chapter 13: Representing Solids on a Paper

Important Concepts and Formulas

1.      A polyhedron is a 3D shape that is formed by polygons.

2.      If in any polyhedron, the line segment joining any two points on its surface lie entirely inside or on the polyhedron, then the polyhedron is called a convex polyhedron.

3.      If the faces of a polyhedron are regular polygons and the same number of faces meet at each vertex, then the polyhedron is called regular polyhedron.

4.      The intersection of two adjacent faces of a solid is called an edge.

5.      A plane surface enclosed by edges is called a face.

6.      A point where three edges meet is called a vertex.

7.      A net of a solid is a plane figure (2D shape) that can be folded up to form the solid (3D shape).

8.      A prism is a polyhedron for which the top and bottom faces are congruent polygons and all other faces are parallelograms.

9.      A pyramid is a polyhedron whose base is a polygon and whose lateral faces are triangles having a common vertex.

10. For a polyhedron, if F is the number of faces, V is the number of vertices and E is the number of edges, then F + V – E = 2. It is called Euler’s formula.

11. A map scale is usually represented as a ratio (for example, 1 : r) or as a fraction (for example, 1/r ).