\[\boxed{\mathbf{696\ (696).\ }Еуроки\ - \ ДЗ\ без\ мороки}\]
\[c_{n} = ( - 1)^{n} \cdot 5\]
\[1)\ c_{1} = - 5\]
\[2)\ c_{8} = 5\]
\[3)\ c_{2k} = ( - 1)^{2k} \cdot 5 = 5 \cdot 1^{k} = 5\]
\[4)\ \ c_{2k + 1} = ( - 1)^{2k + 1} \cdot 5 =\]
\[= ( - 1)^{2k} \cdot ( - 1)^{1} \cdot 5 = - 5\]
\[5)\ \ c_{k + 2} = ( - 1)^{k + 2} \cdot 5 =\]
\[= ( - 1)^{k} \cdot ( - 1)^{2} \cdot 5 = ( - 1)^{k} \cdot 5\]