I have 2 questions:

1) Suppose that f(x,y,z)=(xyz)^(1/5). Show that the directional derivative Duf(0,0,0) exists if and only the unit vector u is a linear combination of some two of the standard unit vectors i, j and k.

2) Define the partial differential derivatives rx and ry of the vector-valued function r(x,y)=ix+jy+kf(x,y) by componentwise partial differentiation. Then show that the vector rx x ry is normal to the surface z= f(x,y).

I dont understand what they are asking