\[\boxed{\mathbf{791\ (791).\ }Еуроки\ - \ ДЗ\ без\ мороки}\]
\[d = - 4;\ \ S_{9} = - 54\]
\[S_{9} = \frac{2a_{1} + 8d}{2} \cdot 9 =\]
\[= \left( a_{1} + 4d \right) \cdot 9\]
\[\left( a_{1} + 4d \right) \cdot 9 = - 54\]
\[a_{1} + 4d = - 6\]
\[a_{1} + 4 \cdot ( - 4) = - 6\]
\[a_{1} = - 6 + 16\]
\[a_{1} = 10\]
\[Ответ:a_{1} = 10.\]
\[\boxed{\mathbf{791.\ }Еуроки\ - \ ДЗ\ без\ мороки}\]
\[1)\ D(y) = \lbrack - 5;6\rbrack.\]
\[2)\ x = - 4;\ - 2;2;4.\]
\[3)\ Возрастает:\lbrack - 5;\ - 3);(0;3);(4;6\rbrack;\]
\[убывает:( - 3;0);(3;4).\]
\[4)\ g(x) < 0:\]
\[x \in \lbrack - 5; - 4) \cup ( - 2;2).\]