[tex] \frac{n!}{(n-8)!} - \frac{n!}{(n-7)!} =9 \frac{n!}{(n-6)!} \\ \\
(n-7)(n-6)(n-5)(n-4)(n-3)(n-2)(n-1)n- \\ (n-6)(n-5)(n-4)(n-3)(n-2)(n-1)n= \\ 9(n-5)(n-4)(n-3)(n-2)(n-1)n \\
(n-7)(n-6)-n+6=9 \\
n^{2} -13n+42-n-3=0 \\
n^{2} -14 n+39=0 \\
rezolvi.ecuatia.cu.delta.si.afli.solitiile[/tex]
