Let 2 be a domain bounded by a closed smooth surface E, and f(r, y, 2) be a scalar function defined on 2 UE. Assume that f has continuous second order partial derivatives and satisfies the Laplace equation on QUE; that is, = 0 on NUE. (i) Let n be the unit normal vector of E, pointing outward. Show that Daj ds = 0. (ii) Let (ro, Y0, 20) be an interior point in 2. Show that 1 f(ro. Ya, 20) cos(r, n) dS, %3! where r = (r ro.y Yo, 2 20) and (r, n) is the angle between the vectors %3D r and n.