Force nominator of a fraction to 1

Hi All,

I have troubles with converting fractions to a fraction with the nominator equal to 1.

I tried functions like "residue" etcetra, but they do not provide the desired result.

As an example, I would like to convert functions like (Ax^2 + Bx + C)/(x^2 + Dx +F) to a function as 1/(....).

Does anybody know a smart way to convert such functions?