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## Homework Statement

I believe this is intended to be a proof of the formula πrl, surface area of a cone.

## Homework 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

## The Attempt at a Solution

When I tried this, I got:

∫2πydx

=2π∫r/hx dx

=2πr/h∫x dx

=2πr/h ∫x

^{2}/2

=πrh! So why has this not worked? I need this to give πrl!