Q. 7

# The base area of a right circular cone is 21m. and height is 14m. Let us calculate the expenditure to colour the curved surface at the rate of ₹1.50 per sq.m.

Let, base radius of the cone = r m

According to problem,

πr2 = 21

r2 = 21 ×7/22

r = 2.58

radius of the base = 2.58 m

Height of the cone = 14 m

slant height of the cone,

= √(2.582 + 142)

= √(6.66 + 196)

= 14.24 cm

curved surface area of the cone,

= πrl

= 22/7 × 2.58 × 14.24

= 115.47 sq. m

Expenditure to colour the curved surface area,

= 115.47 × 1.50

= Rs. 173.2

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The quantity of iron sheet to make boya of right circular conical shape is m2. If the slant height of it is 5m, then let us write, by calculating, the volume of air in the boya and its height. Let us determine the expenditure to colour the whole surface of the boya at the rate of 2.80 per m2. [The width of the iron-sheet not to be considered while calculating.]

West Bengal Board - Mathematics