a^5-2a^4b+a^3b^2-a^3b^2+2a^2b^3-ab^4=(a^5-2a^4b+a^3b^2)-(a^3b^2-2a^2b^3+ab^4)=a^3*(a^2-2ab+b^2)-ab^2*(a^2-2ab+b^2)=a^3(a-b)^2-ab^2(a-b)^2=a(a-b)^2(a^2-b^2)=a(a-b)^2(a+b)(a-b)=a(a-b)^3(a+b)