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# A Hemi-spherical Dome of a Building Needs to Be Painted. If the Circumference of the Base of the Dome is 17.6 Cm, Find the Cost of Painting It, Given the Cost of Painting is Rs. 5 per L00 Cm^2 - CBSE Class 9 - Mathematics

ConceptSurface Area of a Sphere

#### Question

A hemi-spherical dome of a building needs to be painted. If the circumference of the base of
the dome is 17.6 cm, find the cost of painting it, given the cost of painting is Rs. 5 per l00
cm^2

#### Solution 1

Given that only the rounded surface of the dome to be painted, we would need to find the
curved surface area of the hemisphere to know the extent of painting that needs to be done.
Now, circumference of the dome =17.6m.
Therefore, 17.6=2πr.

2× 22/7r= 17.6m.

So, the radius of the dome = 17.6× 7/(2× 22)

m=2.8m

The curved surface area of the dome = 2πr^2

=2× 22/7× 2.8× 2.8cm^2

= 49.28m^2

Now, cost of painting 100cm^2 is Rs. 5.

So, cost of painting 1m^2= Rs.500

Therefore, cost of painting the whole dome
=Rs. 500×49.28
=Rs. 24640 .

#### Solution 2

In the given problem, a hemispherical dome of the building needs to be painted. So, we need to find the surface area of the dome.

Here, we are given the circumference of the hemispherical dome as 17.6 m and as we know that circumference of the hemisphere is given by 2πr. So, we get

2πr = 17.6

2(22/7)r = 17.6

r  = 17.6 (7/22)(1/2)

= 2.8

So, now we find the surface area of the hemispherical dome.

 "surface area" = 2  π r^2

 =2(22/7)(2.8)^2

= 49.28

So, the curved surface area of the dome is 49.28 m2

Since the rate of the painting is given in cm2, we have to convert the surface area from m2 to cm2.

So, we get

Curved surface area = (49.28)(10000)cm^2

= 492800 cm2

Now, the rate of painting per 100 cm2 = Rs 5

The rate of painting per 1 cm2 = 5/100

So, the cost of painting the dome = (5/100) (492800)

=24640

Therefore, the cost of painting the hemispherical dome of the building is Rs 24640 .

Is there an error in this question or solution?

#### APPEARS IN

RD Sharma Solution for Mathematics for Class 9 by R D Sharma (2018-19 Session) (2018 to Current)
Chapter 21: Surface Areas and Volume of a Sphere
Ex.21.10 | Q: 10 | Page no. 8
Solution A Hemi-spherical Dome of a Building Needs to Be Painted. If the Circumference of the Base of the Dome is 17.6 Cm, Find the Cost of Painting It, Given the Cost of Painting is Rs. 5 per L00 Cm^2 Concept: Surface Area of a Sphere.
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