  # Concise Selina Solutions for Class 7 maths Chapter 18 Recognition of Solids (Representing 3D and 2D)

Recognition of Solids: An object that occupies space and has a fixed space is called solid. Some of the characteristics of a solid are: Solids have length, breadth and height. It is a three-dimensional figure. The pyramid is a solid whose base is a rectilinear plane figure such as triangle and whose side faces are triangles with a common vertex.

• If the base of the pyramid is quadrilateral, then it is called a quadrilateral pyramid.
• If the base of the pyramid is a triangle, then it is called a triangular pyramid.

Prism is a solid geometrical figure whose two ends are similar, equal and parallel rectilinear figures and whose sides and faces are either parallelograms or rectangles. It has 3 faces, 9 edges, and 6 vertices. Prism is a solid whose side faces are ||gm and whose end basses are two parallel and congruent polygons. Chapter 18 covers essential concepts like identification of 3D shapes, Euler's formula, nets and mapping space around us. In engineering, a technique called orthographic projection is used to show a three-dimensional object as a two-dimensional drawing. Isometric drawings give 3D views but don't give enough information about an object's true look because you can't see the exact shape and size of each feature.