This might be a really silly question, but I don't understand it.
In finding the electric field due to a thin disk of charge, we use the known result of the field due to a ring of charge and then integrate the relation over the complete radius. But I had a problem in the derivation, as follows:
We assume a ring at a distance $r$ , and of an infinitesimal thickness $dr$ from the center of the disk. Then the very next step in every book I've referred is $dA = 2 \pi rdr$. This is what I don't understand. Shouldn't the area be $$dA = \pi [ (r+dr)^2 - r^2] ~ ?$$ Please help me out here.