- #1

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Suppose we have a differential area element ##dA##. This can be expressed as ##dx \, dy##. However, a change in area ##dA## seems different. Take positions ##x## and ##y## and displace them by ##dx## and ##dy## respectively. Then the change in area ##dA = (x+dx)(y+dy)-xy = xdy+ydx## (ignoring higher order terms). How is the change of area and the differential element different (clearly they must be, right?). Or is it as I've said: one is the CHANGE in surface area and the other is a DIFFERENTIAL area element?

Thanks!