Let $f$ be the function in Exercise $49 .$ Now let $(x, y)$ approach (0,0) along the curves $y=m x^{2}$ and see what happens to $f(x, y)$. That is, find $\lim _{x \rightarrow 0} f\left(x, m x^{2}\right)$. Do you obtain the same limit for every value of $m ?$ What do you now conclude about the continuity of $f$ at (0,0)$?$