



 
Dave, Your mistaken idea of using an average probably comes from finding the area of a trapezoid (namely Area = height of the trapezoid times the average of its parallel sides). Of course, the cross section of a frustum by a plane through its axis is indeed a trapezoid, but you need a bit of calculus to go from there to the volume. You can find the volume easily without calculus. See The correct formula for the volume of a frustum can be found on that page or in our mathcentral files, http://mathcentral.uregina.ca/QQ/database/QQ.09.07/s/gina1.html These formulas tell you that the procedure is more complicated than finding the simple average of the top and bottom areas. Chris
 


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