8. Let f : R^2 ? R be a function defined by
f(x, y) = {
0 for xy ? 0,
1 + x for y = 0,
1 for x = 0,
and let C denote the set of points where f is continuous and D denote the set of points where f is discontinuous.
(a) Find out the sets C and D. Give justification for your answer.
(b) Is the set C an open subset of R^2? Justify your answer.
(c) Is the set D a closed subset of R^2? Justify your answer.
(d) Find out the partial derivatives f_x(0, 0) and f_y(-1, 0).