(1) z=ln(tanx/y)
dz/dx=1/(tanx/y)*(sec²x/y)=sec²x/tanx
dz/dy=1/(tanx/y)*(-tanx/y²)=-1/y
(2) z=arctanx+y/x-y
dz=(dz/dx)*dx+(dz/dy)*dy
=[1/(x²+1)-y/x²]dx+(1/x-1)dy