1. The problem statement, all variables and given/known data I believe this is intended to be a proof of the formula πrl, surface area of a cone. 2. Relevant equations A complete volume of revolution gives you a cone - the height h is the x value on a graph, the radius r is the y value. The y intercept is zero, therefore y=r/hx . All integration is with lim h→0 3. The attempt at a solution When I tried this, I got: ∫2πydx =2π∫r/hx dx =2πr/h∫x dx =2πr/h ∫x2/2 =πrh! So why has this not worked? I need this to give πrl!