Question

slant hight of the frustum of a cone is 5cm and perimeter of the circular ends are 20cm and 8cm, then CSA of the frustum is​

Answer 1

Answer:

70cm²

Explanation:

Given that,

Slant height of the frustum of a cone is 5cm.

And perimeter of both circular ends are 20cm and 8cm.

We know that,

Perimeter of the smaller circle = 2πr

⇒ 8 = 2 × 22/7 × r

⇒ 8 = 44/7 × r

⇒ 8*7/44 = r

⇒ 56/44 = r

⇒ 14/11 = r

And also,

Perimeter of the bigger circular end is given by,

= 2πR

So,

⇒ 20 = 2 × 22/7 × R

⇒ 20 = 44/7 × R

⇒ 20*7/44 = R

⇒ 140/44 = R

⇒ 35/11 = R

Now calculating it's C. S. A. (curved surface area) :-

= 2(r + R)l

Where,

• ‘r’ is the radius of smaller circle = 14/11
• And ‘R’ denotes the radius of bigger circle = 35/11
• ‘l’ is the slant height = 5cm.

So,

= π(r + R)l

= π(14/11 + 35/11) × 5

= π(49/11) × 5

= 22/7 × 49/11 × 5

= 2 × 7 × 5

= 70cm²

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