（1）它们的最简公分母是ab(x-y) x/a(x-y)=xb/ab(x-y) y/b(y-x)=-ab/ ab(x-y)
(2) 它们的最简公分母是（x-y）² a/(y-x)² =a/(x-y)² b/(x-y)(x-y) =b/(x-y)²
(3)它们的最简公分母是 2(a-b) 3a/2a-2b=3a/2(a-b) 2a/b-a =-4a/2(a-b)