Problem with understanding example on Basic Algebraic Geometry (Igor Shafarevich)

Question

On chapter 5, subsection 5.1, Regular Differential 1-forms, in example 3.16 the author gives a manifold given by $x_{0}^3+x_{1}^3+x_{2}^3=0$ and char $ k \neq 3$ if $U_{i,j}$ denotes an open set in which $x_{i},x_j \neq 0$ then he defines $x=x_1/x_0$ and $y=x_2/x_0$ and finally $\phi = dy/x^2$, but i do not know how to differentiate $y$, how does one define such differential?