Definition of functions

Exercise

1.: Find the domain of the following fucntions.
(a): $\displaystyle{f(x,y) = x^2 - y^2}$
(b): $\displaystyle{f(x,y) = \frac{x^2}{x^2 + y^2}}$
(c): $\displaystyle{f(x,y) = \log{(1 - xy)}}$