The height of a cone is 21 cm.Find the area of the base if the slant height is 28 cm.

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Solution

Height of the cone (h)=21 cm

Slant height (l) = 28 cm

$∴ l^{2} = r^{2} + h^{2} \Rightarrow r^{2} = l^{2} - h^{2} = (28)^{2} - (21)^{2} = 784 - 441 = 343 . . . (i) N o w a r e a o f b a s e = π r^{2} = \frac{22}{7} \times 343 [F r o m (i)] = 22 \times 49 = 1078 c m^{2}$

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The curved surface area of a right circular cone whose slant height is 28 cm and base radius is 10.5 cm is

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