Can you please explain how the formula to calculate the Curved Surface Area of a Frustum is 22/7* (R+r)*l ? The way it has been calculate in the solved question seems correct (as in this case the difference of the Curved Surface areas of 2 cones has been taken). But the formula doesnt say so, why?
The proof can be done as I said, using similarity. But it is a long and cumbersome one to type out 🙂 And since I believe that you should not even mug up this formula but rather use similarity wherever possible, I didn’t see a point in going into the details of the proof…
If you really want to know the proof, I would suggest you first try to do it yourself as an exercise (at the very least it will help you practice visualisation); if you fail to get it there are many sources on the web (for example http://www.analyzemath.com/Geometry/conical_frustum.html does it without cumbersome integration-based approaches, in a manner similar to that for the surface area of a cone)
Thank you for the information and also I would like to mention that the content on this web is really helpful. Keep up the good work and please keep posting. Thanks:)
CAT-holics 
Can you please explain how the formula to calculate the Curved Surface Area of a Frustum is 22/7* (R+r)*l ? The way it has been calculate in the solved question seems correct (as in this case the difference of the Curved Surface areas of 2 cones has been taken). But the formula doesnt say so, why?
The proof can be done as I said, using similarity. But it is a long and cumbersome one to type out 🙂 And since I believe that you should not even mug up this formula but rather use similarity wherever possible, I didn’t see a point in going into the details of the proof…
If you really want to know the proof, I would suggest you first try to do it yourself as an exercise (at the very least it will help you practice visualisation); if you fail to get it there are many sources on the web (for example http://www.analyzemath.com/Geometry/conical_frustum.html does it without cumbersome integration-based approaches, in a manner similar to that for the surface area of a cone)
regards
J
Thank you for the information and also I would like to mention that the content on this web is really helpful. Keep up the good work and please keep posting. Thanks:)