原式=[(a+b)/ab(a-b)-4/(a+b)(a-b)]×ab(a-b)/(a-b)²
=[(a+b)²-4ab]/ab(a+b)(a-b)]×ab/(a-b)/(a-b)²
=[(a-b)²/ab(a+b)(a-b)]×ab/(a-b)
=[(a-b)/ab(a+b)]×ab/(a-b)
=1/(a+b)