Show that if $f=0$ on the unit sphere $r=1$ then $$f(0,0,0) = \lim_{\epsilon \to 0}\frac{-1}{4\pi}\iiint_{D_\epsilon}\frac{(x,y,z)\cdot\nabla f}{r^3}\,dV,$$ where $D_\epsilon = \{\epsilon \leq r \leq 1\}$.
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