Area Of Cylinder

Chapter 13 Class 9 Surface Areas and Volumes
Concept wise

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Ex 13.2, 9 Find The lateral or curved surface area of a closed cylindrical petrol storage tank that is 4.2 m in diameter and 4.5 m high. [π΄π π π’ππ π= 22/7] Height (h) of cylindrical tank = 4.5 m Radius (r) of the circular end of cylindrical tank = (4.2/2) m = 2.1 m Curved Surface Area of tank = 2Οrh = (2Γ22/7Γ2.1Γ4.5) m2 = (2Γ22/7Γ21/10Γ45/10) m2 = (2Γ22Γ3/10Γ45/10) m2 = 59.4 m2 Ex 13.2, 9 Find (ii) How much steel was actually used, if 1/12 of the steel actually used was wasted in making the tank. [π΄π π π’ππ π= 22/7] h = 4.5 m , r = 2.1 m Total surface area of tank = (2Οr (r + h)) = 2Γ22/7Γ2.1Γ(2.1+ 4.5) = 2Γ22/7Γ2.1Γ(6.6) = 2Γ22/7Γ21/10Γ66/10 = 87.12 m2 Let Amount of steel used in making the tank = x Steel wasted = π₯/12 Given that Amount of steel required β Amount of steel wasted = TSA of tank x β π₯/12 = 87.12 11/12 π₯ = 87.12 x = 12/11 Γ 87.12 x = 95.04 m2 Therefore, 95.04 m2 steel was used in actual while making such a tank.