**Important Concepts and Formulas**

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 ).