Ex 13.2, 9 - Find (i) The lateral or curved surface area - Area Of Cylinder

  1. Chapter 13 Class 9 Surface Areas and Volumes
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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. [๐ด๐‘ ๐‘ ๐‘ข๐‘š๐‘’ ๐œ‹= 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 = 2ฯ€rh = (2ร—22/7ร—2.1ร—4.5) m2 = (2ร—22/7ร—21/10ร—45/10) m2 = (2ร—22ร—3/10ร—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. [๐ด๐‘ ๐‘ ๐‘ข๐‘š๐‘’ ๐œ‹= 22/7] h = 4.5 m , r = 2.1 m Total surface area of tank = (2ฯ€r (r + h)) = 2ร—22/7ร—2.1ร—(2.1+ 4.5) = 2ร—22/7ร—2.1ร—(6.6) = 2ร—22/7ร—21/10ร—66/10 = 87.12 m2 Let Amount of steel used in making the tank = x Steel wasted = ๐‘ฅ/12 Given that Amount of steel required โ€“ Amount of steel wasted = TSA of tank x โ€“ ๐‘ฅ/12 = 87.12 11/12 ๐‘ฅ = 87.12 x = 12/11 ร— 87.12 x = 95.04 m2 Therefore, 95.04 m2 steel was used in actual while making such a tank.

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