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## Homework Statement

[PLAIN]http://img202.imageshack.us/img202/4535/asdty.jpg [Broken]

## Homework Equations

I think the relevant equations are the divergence theorem

[URL]http://upload.wikimedia.org/math/0/9/b/09b08625d572f4d7c9ceb7585241dc36.png[/URL]

## The Attempt at a Solution

Since it hints at a "clever" solution, I assume the difficult part is to find the right path. I think we can take advantage of the divergence theorem to convert the surface integral to a volume integral (well, it's n-dimensional, so from n-1 -> n).

I think the flux on opposite sides are canceled out, because the function is a function of radius only? But the velocity can be different on the opposite sides.

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