Vc pode usar coordenadas polares:
[tex3]x= r cos \theta \ , \ y=r sen \theta[/tex3]
[tex3]\lim_{(x,y) \to (0,0)} \ \frac{xy^4}{x^2+y^8} = \lim_{r \to 0} \ \frac{r^5 cos \theta sen \theta}{r^2cos^2 \theta+r^8 sen^8 \theta} = \lim_{r \to 0} \ \frac{\cancel{r^5} cos \theta sen \theta}{\cancel{r^2}(cos^2 \theta+r^6sen^8 \theta)}[/tex3]...