f(x, y) = \(\frac{x^2 - y^2}{x^2 + y^2}\) when (x, y) ≠ (0, 0) & 0 when (x, y) = (0, 0) then
1
f(x, y) is continuous at (1, 0)
2
f(x, y) is continuous at (0, 0)
3
f(x, y) is differentiable at (0, 0)
4
all directional derivatives of f(x, y) exist at (0, 0)