I know that the following limit doesn't exist and there's some trick to proving it doesn't exist, anyone know what it is? I think you have to do some substitiution or something. lim x&y -> 0 $\displaystyle (3*x*y^3)/(x^2+y^6)$

I tried showing that the bottom would go to 0 and you can't div by 0, but I got 0 points for the problem with no explanation from the teacher.