(a-b)/(2a+2b)-(a2+b2)/(a2-b2)
=(a-b)/2(a+b)-(a2+b2)/(a+b)(a-b)
=[(a-b)2-2(a2+b2)/[2(a+b)(a-b)]
=(-a2-2ab-b2)/[2(a+b)(a-b)]
=-(a+b)2/[2(a+b)(a-b)]
=-(a+b)/(2a-2b)