$f(x,y)=\frac{xy^3}{x^2 + 4y^2}$,
$(x,y)$ not eaqual to $(0,0)$;
use $\epsilon-\delta$ definition to show that $f(x,y)$ tends to $(0,0)$.
I'm unsure how to deal with fractions for, $\epsilon-\delta$ proof... could applying the proof separately for numerator and denominator; then later combining them help?