Going through this now: pretty straightforward i just want to check that i have covered all aspects including any other approach...
Ok for 15. I have,
##\nabla f= (yz \cos (xyz), xz \cos (xyz), xy \cos (xyz) )##
so,
##D_v f(1,1,1) = \textbf v ⋅\nabla f(1,1,1)##=##\left(\dfrac...