\[\boxed{\text{890\ (890).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[b_{2} = - \frac{1}{32},\ \ b_{3} = \frac{1}{16},\]
\[b_{12} = ?\]
\[\left\{ \begin{matrix} b_{2} = b_{1} \cdot q\ \\ b_{3} = b_{1} \cdot q^{2} \\ \end{matrix} \right.\ \Longrightarrow\]
\[\Longrightarrow \left\{ \begin{matrix} b_{1} \cdot q = - \frac{1}{32} \\ b_{1} \cdot q^{2} = \frac{1}{16} \\ \end{matrix} \right.\ \Longrightarrow\]
\[\Longrightarrow \left\{ \begin{matrix} b_{1} \cdot q = - \frac{1}{32} \\ - \frac{1}{32} \cdot q = \frac{1}{16} \\ \end{matrix} \right.\ \Longrightarrow\]
\[\Longrightarrow \left\{ \begin{matrix} q = - 2\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \\ b_{1} = - \frac{1}{32}\ :q = \frac{1}{64} \\ \end{matrix} \right.\ \]
\[b_{12} = b_{1} \cdot q^{12} = \frac{1}{64} \cdot ( - 2)^{11} =\]
\[= - 32.\]