The formula for the lateral surface of a conical frustum is :
Now, let's consider an infinitesimal conical frustum (we can use it to derive the surface of a cone for example). For $h\rightarrow 0$ , $R_{1}\approx R_{2}$ so $(R_{1} - R_{2})^{2} \rightarrow 0$ and the surface of an infinitesimal conical frustum would be $2\pi R_{2}\,dh$, however i was told that it's not correct. Why?
ps: incredibly this approximation is correct in the case i want to calculate the volume of a cone but not for the surface... :S
