A solid of variable density is in the form of a sphere (x^2 + y^2 + z^2 < a^2, where a>0. The density at the point (x,y,z) is z^2. Use triple integration to find the mass of the solid.
Hence, grad Φ = (y^2, 2xyz-5, xy^2) = (2,7,3) at (3,1,2).
So δΦ/δn = 1/9(1,8,-4).(2,7,3) = 1/9(2+56-12) = 46/9
I understand most of the question (basically from line 2 down), but what I can't work is where the 1/9th came from. Any help please? :l