Chapter 11 Mensuration - Shapes Worksheet Page 12
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M
ATHEMATICS
Did you notice the following:
The cylinder has congruent circular faces that are parallel
to each other (Fig 11.26). Observe that the line segment joining
the center of circular faces is perpendicular to the base. Such
cylinders are known as right circular cylinders. We are only
going to study this type of cylinders, though there are other
Fig 11.26
Fig 11.27
types of cylinders as well (Fig 11.27).
(This is a right
(This is not a right
circular cylinder)
circular cylinder)
THINK, DISCUSS AND WRITE
Why is it incorrect to call the solid shown here a cylinder?
11.7 Surface Area of Cube, Cuboid and Cylinder
Imran, Monica and Jaspal are painting a cuboidal, cubical and a cylindrical box respectively
of same height (Fig 11.28).
Fig 11.28
They try to determine who has painted more area. Hari suggested that finding the
surface area of each box would help them find it out.
To find the total surface area, find the area of each face and then add. The surface
area of a solid is the sum of the areas of its faces. To clarify further, we take each shape
one by one.
11.7.1 Cuboid
Suppose you cut open a cuboidal box
and lay it flat (Fig 11.29). We can see
a net as shown below (Fig 11.30).
Write the dimension of each side.
You know that a cuboid has three
pairs of identical faces. What
expression can you use to find the
area of each face?
Fig 11.29
Fig 11.30
Find the total area of all the faces
of the box. We see that the total surface area of a cuboid is area I + area II + area III +
area IV +area V + area VI
= h × l + b × l + b × h + l × h + b × h + l × b
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