(х^2-2х+15)/(х+3)(х+4)=(х^2-2х+15)/(x^2+4x+3x+12)=
=(х^2-2х+15)/(x^2+7x+12)=(х^2+7x+12-9х-3)/(x^2+7x+12)=
=1-(9x+3)/(x^2+7x+12)
(х^2-2х+15)/(х+3)(х+4)=(х^2-2х+15)/(x^2+4x+3x+12)=
=(х^2-2х+15)/(x^2+7x+12)=(х^2+7x+12-9х-3)/(x^2+7x+12)=
=1-(9x+3)/(x^2+7x+12)