\(\frac{y^3-x^3}{x^4-y^4}=\frac{\left(y-x\right)\left(y^2+xy+x^2\right)}{\left(x^2-y^2\right)\left(x^2+y^2\right)}=\frac{-\left(x-y\right)\left(y^2+xy+x^2\right)}{\left(x-y\right)\left(x+y\right)\left(x^2+y^2\right)}=\frac{-\left(y^2+xy+x^2\right)}{\left(x+y\right)\left(x^2+y^2\right)}\)