Question

slant height of a frustum of a cone is 4 cm and the perimeter of its circular ends are 18 cm and 6 cm find the curved surface area of frustum ​

Answer 1

Answer:

Given,

2πr=18

22/7r=9

r=63/7

Similarly, R=21/22

Now, CSA of frustum=πl(r+R)

=4×22/7(63/22+21/22)

=88/7×84/22

=4×12

=48cm²

Answer 2

Given,

Slant height of frustum of cone (l) = 4 cm

Let ratio of the top and bottom circles be r1 and r2

And given perimeters of its circular ends as 18 cm and 6 cm

⟹ 2πr1 = 18 cm; 2πr2 = 6 cm

⟹ πr1= 9 cm and πr2 = 3 cm

We know that,

Curved surface area of frustum of a cone = π(r1 + r2)l

= (πr1+πr2)l = (9 + 3) × 4

 = (12) × 4 = 48 cm2

Therefore, the curved surface area of the frustum = 48 cm2.