(123/456)

r = (((x^3) - 2  (x^2) - 5  x + 6)/(x - 1))

Together[r]

Together[(((x^10) - (y^10))/((x^4) - (y^4)))]

Together[
      (x/((x - y) (x - z))) + (y/((y - z) (y - x))) + 
        (z/((z - x) (z - y)))]