\begin{equation}
\left\{
    \begin{array}{c}
    I_1\times R_1+I_3\times R_3+I_6\times R_6-U_{input}=0\\
    I_2\times R_2+I_5\times R_5+I_8\times R_8-U_{input}=0\\
    I_2\times R_2+I_4\times R_4+I_7\times R_7-U_{input}=0\\
    \end{array}
\right.
\end{equation}
