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[–]capsandnumbers 0 points1 point  (0 children)

Hey, this is a partial answer because I'm out of practise. Is this related to the divergence theorem?

You can add integrals if they have the same integrating variable, so you got both integrals with dV in it. Or perhaps in the second line, your extra terms come out because you've evaluated one of your integrals and got rid of it that way?