Q11 of 24 Page 227

The external diameter of a conical-coronet made off thermocol is 21cm. in length. To wrap up the outer surface of the coronet with foil, the expenditure will be ₹57.75 at the rate of 10p. per cm2. Let us write by calculating, the height and slant height of the coronet.

External diameter of coronet = 21 cm


∴ external radius of coronet, r = 21/2 = 10.5 cm


Let, slant height = l cm


Height = h cm


According to problem,


⇒πrl = 57.75/0.1 [10p. = Rs. 0.1]


⇒ 22/7 × 10.5 × l = 577.5


⇒ l = 577.5 × 7/(22 × 10.5)


⇒ l = 17.5


∴ slant height = 17.5 cm


∴ height = √(17.52 – 10.52) = √(306.25 – 110.25) = 14 cm


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