\[b_{n} = n^{2} - 4n + 9\]
\[b_{n} = 59:\]
\[n^{2} - 4n + 9 = 59\]
\[n^{2} - 4n - 50 = 0\]
\[D_{1} = 2 + 50 = 52\]
\[n_{1,2} = 2 \pm \sqrt{52}.\]
\[Ответ:не\ является.\]