Q9 of 48 Page 270

If the radii of a frustum of a cone are 7 cm and 3 cm and the height is 3 cm, then the curved surface area is …………cm2.

In frustum


(Slant height)2 = (difference in radii )2 + (height)2


(Slant height)2 = (R - r)2 + (height)2


(Slant height)2 = (7cm – 3cm)2 + (3 cm)2


(Slant height)2 = 16 cm2 + 9 cm2


(Slant height)2 = 25 cm2


Slant height = √ (25 cm2)


Slant height = 5 cm


CSA of frustum = π[R + r]l


= π[7cm + 3cm] × 5cm


= 50 π cm2

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