# The slant height

Given: perimeter of upper circle = 18 cm

Perimeter of lower circle = 6 cm

Slant height of frustum = l = 4 cm

Formula: Total surface area of frustum = πr2 + πR2 + π(R + r)l cm2

Let r be the radius of lower circle and R be the radius of upper circle

Now perimeter of circle = circumference of circle = 2π × radius

Perimeter of upper circle = 2πR

18 = 2 × π × R

R = 9/π cm

Perimeter of lower circle = 2πr

6 = 2 × π × r

r = 3/π cm

Now we have asked curved surface area, so we should subtract the top and bottom surface areas which are flat circles.

Surface area of top = πR2

Surface area of bottom = πr2

Curved surface area = total surface area - πr2 - πR2 cm2

= πr2 + πR2 + π(R + r)l - πr2 - πR2 cm2

= π(R + r)l cm2

= π × [(9/π) + (3/π)] × 4 cm2

= (9 + 3) × 4 cm2

= 48 cm2

curved surface area = 48 cm2

Rate this question :

How useful is this solution?
We strive to provide quality solutions. Please rate us to serve you better.
Related Videos
Trigo Marathon Part 245 mins
Trigo Marathon Part 146 mins
Application of Trigo Important Questions44 mins
Arithmetic Progression and 'nth' Term of AP55 mins
Chemical Properties of Metal and Non Metal61 mins
Extraction of Metal56 mins
Purification of Metals60 mins
Properties of Metal and Non Metal59 mins
Introduction of Metal and Non Metal59 mins
Outcomes of Democracy43 mins
Try our Mini CourseMaster Important Topics in 7 DaysLearn from IITians, NITians, Doctors & Academic Experts
Dedicated counsellor for each student
24X7 Doubt Resolution
Daily Report Card
Detailed Performance Evaluation
view all courses