(a^4-b^4)/(a^2+b^2)=
=(a^2-b^2)(a^2+b^2)/(a^2+b^2)=a^2-b^2;
(x^4-y^4)/(x^2-y^2)=
=(x^2-y^2)(x^2+y^2)/(x^2-y^2)=x^2+y^2;
(a^-b^3)/(a^4-b^4)=
=(a-b)(a^2+ab+b^2)/(a-b)(a+b)(a^2+b^2)=
=(a^2+ab+b^2)/(a+b)(a^2+b^2).
(a^2+ab+b^2)/(a^3-b^3)=
=(a^2+ab+b^2)()/(a-b)(a^2+ab+b^2)=
=1/(a-b).