Q9 of 67 Page 193

The slant height and radius of a cone are in the ratio 7 : 4. If its curved surface area is 792 cm2, find its radius.

Let the slant height of cone be l and radius of base of cone be r


Ratio of slant height to radius = 7:4


⇒ =


⇒ l = × r


Curved surface area of cone = 792 cm2


⇒ πrl = 792


put l = × r


⇒ × r × × r = 792


⇒ × r2 = 792


⇒ r2 =


⇒ r2 = 36 × 4


⇒ r2 = 144


⇒ r2 = √144


⇒ r = ± 12


r is radius and radius cannot be negative thus r = 12 cm


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