documentclass[11pt,]{book}
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begin{document}
setcounter{chapter}{1}
chapter{Vector geometry}

section{Affine spaces}
In Chapter 1, we explained how Euclid built up the whole of geometry from a small number of fundamental assumptions or emph{axioms}. Following Euclid’s method, we stated nine axioms and from them developed the theory of emph{vectors}.

end{document}

documentclass{book}
usepackage{titlesec}
titleformat{chapter}[display]{}{hrule}{0pt}{centerbfseriesthechapter\[3ex]}[endcenterhrule]
begin{document}
chapter{test}
alkasdjalsk laskj laskj ldsakj dl
end{document}

documentclass{book}
usepackage{titlesec}
titleformat{chapter}[display]{}{hrule}{0pt}{centersffamilythechapter\[3ex]}[endcenterhrule]
begin{document}
chapter{test}
alkasdjalsk laskj laskj ldsakj dl
end{document}