(k) (gradf) Ã (div F) 13-18 Determine whether or not the vector field is conservative. If it is conservative, find a function f such that F Vf. 13. F(x, y, z) y":31+ 2xyz3jt 3xy2z-k 14. F(x, y, z) = xyz? i + x2yz2jt x2yzk 15. F(t, y, 2) 3xy22 i +2x2yz3j 3x2y'22 k 16. F(x, y, z)-i+ sin z j +y cos z k (1) div(curÅ(grad f)) 18. F(x, y, z) = ex sin ye i + ze" cos yzj + ye"cos yz k 19. Is there a vector field G on R3 such that