(x^2-18x+81) / (81-x^2) =

( (х - 9) * (x - 9)) / ( (9 - x) * (9 + x) ) =

( - ( x - 9) * (9 - x) ) / ( (9 - x) * (9 + x) ) =

(9 - x) / (9 + x) .