To evaluate the following limit:
$$\lim _ { ( x , y , z ) \rightarrow ( 0 , 0 , 0 ) } \frac { x ^ { 2 } + y ^ { 4 } + z ^ { 6 } } { x + y ^ { 2 } + z ^ { 3 } }$$
Is it reasonable to set $x=y^5$ and $z=-y^{\frac{2}{3}}$ and draw the conclusion that the limit does not exist?
I also tried to use the spherical coordinate and get $\frac{0}{\sin(\phi)\cos(\theta)}$ which I'm not sure how to deal with?