help with $\lim\limits_{(x,y) \to (0,0)} f(x,y) = {\cos(x) -1 - {x^2/2} \over x^4 + y^4}$ $\lim\limits_{(x,y) \to (0,0)} f(x,y) = \dfrac{\cos(x) -1 - {x^2 \over 2}}{x^4 + y^4}$
Is the following approach correct?
If we approach the origin from $y$ , that is $x = 0$:
$\lim\limits_{(x=0,y) \to (0,0)} f(0,y) = {0 \over y^4} = 0$
Now we approach the origin from $x$ and use $\cos(x) \sim_{0} 1 - {x^2\over 2}$
$\lim\limits_{(x,y=0) \to (0,0)} f(0,y) = { -x^2 \over x^4} =- {1 \over x^2}=-\infty$
Then we can conclude that the limit does not exist.
My teacher took another approach where he uses the $\cos (2x)$ formula and ends up using $\lim\limits_{u\to0}\dfrac{\sin u}{u}=1$ so I'm wondering if my solution is OK or not.

Use the straight line $y=mx$ for $m \in \mathbb{R}$:
$$
\lim_{x \to 0} \frac{\cos x -1 -\frac{x^2}{2}}{(1+m^2)x^2} = \lim_{x \to 0} \frac{1-\frac{x^2}{2}+\frac{c^4}{4!}-1-\frac{x^2}{2}}{(1+m^2)x^2}=\lim_{x \to 0} \frac{-x^2+\frac{c^4}{4!}}{(1+m^2)x^2}
$$
for some $c \in (0,x)$. Therefore
$$
\lim_{x \to 0} \frac{-x^2+\frac{c^4}{4!}}{(1+m^2)x^2} = -\frac{1}{1+m^2},
$$
which clearly depends on $m$. The original limit cannot exist.
