3.(a) Use the Divergence Theorem to find the flux of F across the surface o with outward orientation for F(x, y,z) = zi - x j+ y k where o is the sphere x2 + y? + z? = a2. (b) Evaluate the surface integral f(x, y, z)ds where f (x, y, z) = xy; o is the portion of the plane x + y + z = 1 lying in the first octant