\documentclass[12pt,a4paper]{book}
\usepackage[cp1250]{inputenc}
\usepackage[polish]{babel}
\usepackage[MeX]{polski}    %full polonization
\usepackage[OT4]{fontenc}
\usepackage{calc}
\usepackage{graphicx}
\usepackage[a4paper,left=2.5cm,right=2.5cm,top=2.5cm,bottom=2.5cm]{geometry}
\usepackage{amsmath}
\begin{document}
            \begin{equation}
                u_{1}=d_{1}-x, \hspace{1cm} u_{i}=d_{i}-x-\frac{e_{i-1}^{2}}{u_{i-1}} \quad (i=2,\ldots,n-1),~\newline
                u_{N}=d_{N}-\sum_{i=1}^{n-2}\frac{|s_{i}|^{2}}{u_{i}} - x -\frac{|e_{N-1}+s_{N-1}|^{2}}{u_{N-1}},~\newline
                s_{1}=e_{0}, \hspace{1cm} s_{i}=-\frac{e_{i-1}s_{i-1}}{u_{i-1}} \quad(i=2,\ldots,n-1)
            \end{equation}
\end{document}