(1) their simplest common denominator is ab (x-y) x/a (x-y) = XB/ab (x-y) y/b (y-x) =-ab/ab (x-y).

(2) Their simplest common denominator is (x-y)? a/(y-x)？ =a/(x-y)？ b/(x-y)(x-y) =b/(x-y)？

(3) Their simplest common denominator is 2 (a-b) 3a/2a-2b = 3a/2 (a-b) 2a/b-a =-4a/2 (a-b).