Use Green's first identity to show ∭|∇f|²dV = ∬(df/dn) dS.
Answer: Let be a closed surface and be the region bounded by the surface . Let be the normal vector to the surface . Then, it follows from the definition of the directional derivative that
It also follows from the definition of the gradient and divergence that
Now, by Gauss' divergence theorem (Green's first identity), it follows that
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