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# Vector / Integral question - probably simple, but can't see answer. Watch

1. http://www.maths.ox.ac.uk/system/fil.../Sheet3_13.pdf

It's question 6 of the above, just the bit at the start of step c) where you prove .

Basically, you have , and you can assume the identities proved in a) and b). Prime on a vector operator means treat r' as variable and r as constant.

What I did was;

Then using integration by parts or the product rule;

However, this is the wrong answer, as the first term won't always be zero and the second term is the required integral. The class notes I copied off the board (without understanding at the same time) seem to do the same thing I have, but the answer is definitely the one asked for, as that's what it says in the textbook.

Can anyone tell me what I'm missing. I don't know where to start looking. Thanks

EDIT: I've solved the problem - there is a typo in the question. It should say rho vanishes outside and on the boundary, which causes the extra term to disappear, by the divergence theorem. I am now really happy. Feel free to delete the thread.

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Updated: May 16, 2013
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