n^3-5n+9=(n^2-n-4)(n+1)+13,
(n^3-5n+9)/(n+1)=n^2-n-4 + 13/(n+1);
n+1=1, n=0∉N,
n+1=13, n=12,
f(12)=(12^3-5*12+9)/(12+1)=1677/13=129.
n^3-5n+9=(n^2-n-4)(n+1)+13,
(n^3-5n+9)/(n+1)=n^2-n-4 + 13/(n+1);
n+1=1, n=0∉N,
n+1=13, n=12,
f(12)=(12^3-5*12+9)/(12+1)=1677/13=129.