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\documentclass[12pt,a4paper,oneside]{report}
\linespread{1.3}
\usepackage[utf8]{inputenc}
\usepackage{polski}
\usepackage[T1]{fontenc}
\usepackage{indentfirst}
\usepackage{amsthm}
\usepackage{amssymb}
\usepackage{color}
\newtheorem{tw}{Twierdzenie}[chapter]
\newtheorem{lem}{Lemat}[chapter]
\newtheorem{wn}{Wniosek}[chapter]
\theoremstyle{definition}
\newtheorem{deff}{Definicja}[chapter]
%\renewcommand{\thedeff}{\!\!}
\newtheorem{ft}{Fakt}[chapter]
\newtheorem{ex}{Przykład}[chapter]
\newtheorem{uw}{Uwaga}[chapter]
\newtheorem{zad}{Zadanie}[chapter]
\newtheorem{wl}{Własność}[chapter]
\usepackage{graphicx}
\usepackage{geometry}
\usepackage{enumerate}
\geometry{vmargin=2.5cm, inner=3.0cm, outer=2.5cm, includehead, includefoot}
\usepackage{amsfonts}
\usepackage[centertags]{amsmath}
\numberwithin{equation}{chapter}
\usepackage{fancyhdr}
\usepackage{ifsym}
\pagestyle{fancy}
\fancyhead[L,RO]{\thepage}
\renewcommand{\headrulewidth}{0pt}
\renewcommand{\qedsymbol}{ckd.}
\usepackage{titlesec}
\titleformat{\chapter}[display]
{\normalfont\Large\bfseries\center}{\chaptertitlename\ \thechapter}{0pt}{\Large}
\titlespacing*{\chapter}{0pt}{0pt}{40pt}
\setlength\arraycolsep{2pt}
\makeatletter
\newcommand{\labitem}[2]{
\def\@itemlabel{\textbf{#1}}
\item
\def\@currentlabel{#1}\label{#2}}
\makeatother
\titleformat{\section}
{\normalfont\large\bfseries}{\thesection}{1em}{\large}