Good info here – cones can be gnarly to work with. Those formulas are not easy to memorize. I’ve always found it easier to remember how to derive the formulas from scratch than to remember them outright. For example, I get the slant height of a frustum (or a cone, for that matter) by remembering that it comes from the Pythagorean Theorem. From there, it comes easy. Thanks!
That is not an implementation of the formula, but a totally separate approach. Note that the L is different there. In the formula it is the slant height of the frustum. In the other case the L and l are the slant heights of the two cones. If you try deriving the formula to the frustum, you will see that the result is equivalent.
CAT-holics 
Good info here – cones can be gnarly to work with. Those formulas are not easy to memorize. I’ve always found it easier to remember how to derive the formulas from scratch than to remember them outright. For example, I get the slant height of a frustum (or a cone, for that matter) by remembering that it comes from the Pythagorean Theorem. From there, it comes easy. Thanks!
PERFECT..KEEP POSTING PLZ
here CSA of frustrum is [pie (R L+ rL)] but in its latter post (its implementation) you have used pie(R-r)L, which one is correct ?
That is not an implementation of the formula, but a totally separate approach. Note that the L is different there. In the formula it is the slant height of the frustum. In the other case the L and l are the slant heights of the two cones. If you try deriving the formula to the frustum, you will see that the result is equivalent.
regards
J