c1=-31 и c6=-11?
\[c_{1} = - 31;\ \ c_{6} = - 11:\]
\[c_{6} = c_{1} + 5d\]
\[5d = c_{6} - c_{1}\]
\[5d = - 11 + 31\]
\[5d = 20\]
\[d = 4.\]
\[c_{n} = 5:\]
\[c_{n} = c_{1} + d(n - 1)\]
\[d(n - 1) = c_{n} - c_{1}\]
\[4 \cdot (n - 1) = 5 + 31\]
\[n - 1 = 36\ :4\]
\[n - 1 = 9\]
\[n = 10.\]
\[Ответ:да,\ является;\ \ c_{10} = 5.\]