a^4-b^4
________
ab^2-a^3

(a^4-b^4)/(ab^2-a^3)=((a^2-b^2)(a^2+b^2))/(a(b^2-a^2))=-((a^2-b^2)(a^2+b^2))/(a(a^2-b^2))=-(a^2+b^2)/a