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$$4\pi(r+dr)^2-4\pi r^2=4\pi r^2+8\pi rdr+4\pi (dr)^2-4\pi r^2=8\pi rdr+4\pi (dr)^2$$And now I'm stuck. I think the second term can be ignored because it's so small, but how to deal with the fact that it's 4 times more than ##2\pi rdr##? Can someone explain this to me or tell me why I shouldn't think this way, where did I go wrong?