ICSE Class 8 Maths Selina Solutions Chapter 19: Chapter 19 of the ICSE Class 8 Maths Selina Solutions focuses on drawing three-dimensional (3-D) objects on two-dimensional (2-D) paper. It explains different ways to draw 3-D shapes using special paper with dots and shows how to view and sketch objects from different angles like the top, front, and side views.
The exercises help students develop the skill to picture and draw complex shapes, making this chapter a key part of their math studies.ICSE Class 8 Maths Selina Solutions Chapter 19 PDF
Question 1.
If a polyhedron has 8 faces and 8 vertices, find the number of edges in it.Solution:
Faces (F) = 8, Vertices (V) = 8 Using Euler’s formula, F + V – E = 2 8 + 8 – E = 2 16 − E = 2 E 16 − 2 ⇒ E = 14 Therefore, there are 14 edges in a polyhedron.Question 2.
If a polyhedron has 10 vertices and 7 faces, find the number of edges in it.Solution:
We have to find the number of edges of the given polyhedron . Given, number of faces F = 7 Number of Vertices V = 10 Euler’s formula for any polyhedron is, F + V – E = 2 Where F stands for number of faces, V for number of vertices E for number of edges. According to the question, 7 + 10 - E = 2 17 - E = 2 17 - 2 = E E = 15 Therefore, the number of edges is 15.Question 3.
State, the number of faces, number of vertices and number of edges of: (i) a pentagonal pyramid (ii) a hexagonal prismSolution:
(i) A pentagonal pyramid Number of faces = 6 Number of vertices = 6 Number of edges = 10 (ii) A hexagonal prism Number of faces = 8 Number of vertices = 12 Number of edges = 18Question 4.
Verily Euler’s formula for the following three dimensional figures:Solution:
(i) Number of vertices = 6 Number of faces = 8 Number of edges = 12 Using Euler formula, F + V - E = 2 8 + 6 - 12 = 2 2 = 2 Hence proved. (ii) Number of vertices = 9 Number of faces = 8 Number of edges = 15 Using, Euler's formula, F + V - E = 2 9 + 8 - 15 = 2 2 = 2 Hence proved. (iii) Number of vertices = 9 Number of faces = 5 Number of edges = 12 Using, Euler's formula, F + V - E = 2 9+5-12=2 2 = 2 Hence proved.Question 5.
Can a polyhedron have 8 faces, 26 edges and 16 vertices?Solution:
Number of faces = 8 Number of vertices = 16 Number of edges = 26 Using Euler's formula F + V − E = 2 But 8 + 16 − 26 ≠ 2 ⇒ − 2 ≠ 2 No, a polyhedron cannot have 8 faces, 26 edges and 16 vertices.Question 6.
Can a polyhedron have: (i) 3 triangles only ? (ii) 4 triangles only ? (iii) a square and four triangles ?Solution:
(i)No, polyhedron has three faces. (ii)Yes, tetrahedron has four triangles its faces. (iii)Yes, a square pyramid has a square as its base and four triangles as its faces.Question 7.
Using Euler’s formula, find the values of x, y, z.Solution:
(i) F + V − E = 2 ⇒ x + 15 − 20 = 2 ⇒ x − 5 = 2 ⇒ x = 2 + 5 = 7 (ii) F + V − E = 2 ⇒ 15 + y − 26 = 2 ⇒ y − 11 = 2 ⇒ y = 2 + 11 y = 13 (iii) F + V − E = 2 ⇒ 14 + 26 − Z = 2 ⇒ − Z = 2 − 40 ⇒ Z = 38Question 8.
What is the least number of planes that can enclose a solid? What is the name of the solid.Solution:
The least number of planes that are required to enclose a solid is '4'. The name of the solid is tetrahedron. It is a solid with four planes.
Solution:
Compare a square prism and a cube :
Hence, a square prism is same as a cube.
Question 10.
A cubical box is 6 cm x 4 cm x 2 cm. Draw two different nets of it.Solution:
Solution:
Question 12.
Name the polyhedron that can be made by folding each of the following nets:Solution:
(i) Triangular prism. It has 3 rectangles and 2 triangles. (ii) Triangular prism. It has 3 rectangles and 2 triangles. (iii) Hexagonal pyramid as it has a hexagonal base and 6 triangles.Question 13.
Draw nets for the following polyhedrons:Solution: