(a+b)^2(a-b)^2=[(a+b)(a-b)]^2=(a^2-b^2)^2=a^4-2a^2b^2+b^4
(a+b)^2(a-b)^2=(a+b)(a+b)(a-b)(a-b)=(a^2-b^2)(a^2-b^2)=(a^2-b^2)^2
=((a+b)*(a-b))^2=(a^2-b^2)^2=a^4+b^4-2a^2*b^2
=((a+b)(a-b))^2=(a^2-b^2)^2=a^4-2a^2*b^2+b^4