Evaluate the surface integral s f · ds for the given vector field f and the oriented surface s. in other words, find the flux of f across s. for closed surfaces, use the positive (outward) orientation. f(x, y, z) = −xi − yj + z3k, s is the part of the cone z = x2 + y2 between the planes z = 1 and z = 2 with downward orientation.