 # NCERT Solutions Class 9 Mathematics Solutions for Surface Areas and Volumes - Exercise 13.2 in Chapter 13 - Surface Areas and Volumes

The inner diameter of a circular well is 3.5 m. It is 10 m deep. Find

(i) its inner curved surface area,

(ii) the cost of plastering this curved surface at the rate of Rs. 40 per \mathbf{m}^{2}

(Assume π=22/7)

Inner radius of circular well, r = 3.5/2m = 1.75m

Depth of circular well, say h = 10m

(i)Inner curved surface area = 2πrh

= ( 2 x 22/7 x 1.75 x 10)

= 110

Therefore, the inner curved surface area of the circular well is 110 \mathrm{m}^{2}

(ii)Cost of plastering 1 m area = Rs.40

Cost of plastering 110 m area = Rs (110 x 40) = Rs.4400

Therefore, the cost of plastering the curved surface of the well is Rs. 4400.

Video transcript
"hello students welcome to lido q a video session i am your math tutor and question for today is the inner diameter of the circular well is 3.5 meter and it's the 10 meter deep find first its inner cow surface area second the cost of plastering this crowd surface area at the rate of rupees 40 per meter square assume pi is equal to 22 by 7 now here to begin with let us see what is given the inner radius of the circular wall is given in the form of diameter so here radius can be found with the help of diameter 3.5 meter diameter by 2 will be radius so radius is equal to diameter upon 2 that is 3.5 upon 2 and it is 1.75 meter now depth of the circular well say h the h will be 10 meter which is given so height of well h emitter now the first thing we need to find is the inner curve surface area formula to find the inner curve surface area of the cylinder is 2 pi rh val is cylindrical in shape basically so 2 pi r h 2 into 22 by 7 into 1.75 into 10 so curves inner curve surface area is 110 meter square therefore this is the inner curve surface area now the next thing we need to find is the cost of plastering this curve service area at the rate of rupees 40 per meter square so here rate of fast plastering one meter square is rupees 40 so how much will be the rate of plastering 40 meter square so for this we have to multiply one meter cost with the amount of area to be plastered so 40 into 110 so rate will be getting as 4400 rupees that is our final answer so if you have any query regarding this you can post it in our comment section and subscribe to lido for more such q a thank you for watching "
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