目测是要证明b/(a+1)-a/(b+1) = 2(b-a)/(a+b+1).
b/(a+1)-a/(b+1)
= (b(b+1)-a(a+1))/((a+1)(b+1))
= (b-a)(a+b+1)/(1+a+b+ab)
= (b-a)(a+b+1)²/((1+a+b+ab)(a+b+1))
= (b-a)(a²+b²+1+2a+2b+2ab)/((1+a+b+ab)(a+b+1))
= (b-a)(2+2a+2b+2ab)/((1+a+b+ab)(a+b+1))
= 2(b-a)/(a+b+1).