NCERT Class 9 Solutions: Surface Areas and Volumes (Chapter 13) Exercise 13.1 – Part 2

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Q-4 The paint in a certain container is sufficient to paint an area equal to . How many bricks of dimensions can be painted out of this container?

Box: Rectangular Prism with surface area A=2(wh+lw+lh)

Box: Rectangular Prism

Box: Rectangular Prism with surface area A=2(wh+lw+lh)

  • The container can paint an area

  • Dimensions of brick

Therefore, total surface area of a brick

Number of bricks can be painted =

Q-5 A cubical box has each edge and another cuboidal box is long, wide and high.

  1. Which box has the greater lateral surface area and by how much?

  2. Which box has the smaller total surface area and by how much?

Solution:

Surface area of cuboid is 2(lw+hw+lh)

Surface Area of Cuboid

Surface area of cuboid is 2(lw+hw+lh)

  • Cubical box has each edge

  • Length

  • Width

  • Height

Lateral surface area is the area of the faces along the height. That is, it is the area excluding the top and bottom. Now for cube this would be the area of any 4 faces. For the cuboid it would be

Solution (i)

  • Lateral surface area of cubical box of edge

  • Lateral surface area of cuboid box is

  • Thus, lateral surface area of the cubical box is greater by

Total surface area of cubical box of edge

Total surface area of cuboidal box

=

=

=

So, total surface area of cubical box is smaller by

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