Question

let E be a simple solid region, and let S be a boundary surface of E. Given with positive outward orientation. assume (0,0,0) not an element of S.
Show that the double integral r/||r||^3 .dS =
(4 pi if (0,0,0) element of E
0 IF (0,0,0) not element of E )

where r= and ||r||= sqrt(x^2+y^2+z^2) (not r has a vector sign)