Discussion:
Q: If a function f of one variable is differentiable at x_0, it has a derivative at x_0 (via the limit definition of derivative). As one consequence, we know that f is continuous at x_0. This is not necessarily so in the case of a function of two or more variables. Consider the function:
f(x,y) = (xy^2) / (x^2 + y^4) where (x,y) =/ (0,0)
f(x,y) = 0 where (x,y) = (0,0)
Show that f(x,y) is not continuous at (0,0) by showing that lim f(x,y) as (x,y) --> (0,0) does not exist.