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\documentclass{scrbook}
\usepackage{pslatex}
\begin{document}
\[ \eta \sin\mathnormal{\eta}  \]
Der Winkel $ \eta $ soll ebenfalls kursiv dargestellt werden.
\end{document} 

%------------------------------------------------------------------------------------
\documentclass[   a4paper,
            12pt,
            titlepage,
            twoside,
            smallheadings,
            DIV            = 14,
            BCOR         = 6mm,
            parskip         = half,
            numbers       = noenddot,
            headings       = small,
            cleardoublepage = empty]{scrbook}
%------------------------------------------------------------------------------------
% Laden der Packages
%------------------------------------------------------------------------------------

%------------------------------------------------------------------------------------
\usepackage[ansinew]{inputenc}  
% Umlaute ""ä, ö, ü"" im Fließtext eintippen zu können
%Alternativ: \"A or {\"A}

\usepackage{soul}         % Texthervorhebung: Fett, Unterstreichen
     \setul{0.4ex}{0.2ex}        % Unterstreichen-Einstellung 01
                       % \setul{Position, +runter, -hoch}{Dicke}
     % \setuldepth{g}         % Unterstreichen-Einstellung 02
%-----------------------------------------------------------------------------------

\usepackage{ae}                  % almost european, virtueller T1-Font
\usepackage{array}              % für aufwändigere Tabellen
\usepackage{colortbl}                  % farbige Tabellen (v. D. Carlisle)
\usepackage{longtable}         % seitenübergreifende Tabellen
\usepackage{tabularx}       % automatische Spaltenbreite
         \renewcommand{\arraystretch}{1.3}
\usepackage{color}          % farbiger/grauer Text

\usepackage[british,ngerman]{babel}      
% PROBLEM mit package(inputenc)

\usepackage{amsfonts}
\usepackage{amsmath,amssymb}
\usepackage[sc]{mathpazo}      % Palatino Schrift
         \linespread{1.05}
\usepackage[T1]{fontenc}
\renewcommand*\familydefault{\rmdefault}
\usepackage[automark,nouppercase,headsepline]{scrpage2}
\usepackage{pdfpages}
\usepackage[T1,hyphens]{url}    % much like \ verb allow line breaks for paths and URLs
\usepackage{calc}
\usepackage{ifthen}
\usepackage{lscape}
\usepackage{graphicx}
\usepackage{tocbasic}
\usepackage{paralist}
\usepackage{booktabs}
\usepackage{multirow}
\usepackage{multicol}
\usepackage{rotating}
\usepackage{mathtools}
\usepackage{mdwlist}
\usepackage{anysize}
\usepackage{pslatex}
\usepackage{wrapfig}
\usepackage{scrhack}
\usepackage{csquotes}
\usepackage{xkeyval}
\usepackage{xfor}
\usepackage{amsgen}
\usepackage{xfrac}
\usepackage[cmyk,dvipsnames]{xcolor}
\allowdisplaybreaks[1]
\usepackage{tikz}            

\usepackage{fixmath}
%------------------------------------------------------------------------------------


\begin{document}

\chapter{Kapitel 02}

...
\\ \\

$  A = 1 + \dfrac{b}{c}  $

\vspace{0,50cm}

$  D = \dfrac{10 + cos\eta}{2}  $

\vspace{0,50cm}

mit $  sin\mathnormal{\eta} = \dfrac{E_1}{E_2}  $

\vspace{0,50cm}

\textcolor{red}{\textbf{Der Winkel $ \eta $ soll ebenfalls kursiv dargestellt werden.}}


\end{document}

\documentclass[   a4paper,
            12pt,
            titlepage,
            twoside,
            smallheadings,
            DIV            = 14,
            BCOR         = 6mm,
            parskip         = half,
            numbers       = noenddot,
            headings       = small,
            cleardoublepage = empty]{scrbook}

\usepackage{amsmath,amssymb}
\usepackage[british,ngerman]{babel}
\usepackage{amsfonts}
\usepackage{amsmath,amssymb}
\usepackage[sc]{mathpazo}\linespread{1.05}
\usepackage[T1]{fontenc}
\usepackage{calc}
\usepackage{ifthen}
\usepackage{lscape}
\usepackage{graphicx}
\usepackage{tocbasic}
\usepackage{paralist}
\usepackage{booktabs}
\usepackage{multirow}
\usepackage{multicol}
\usepackage{rotating}
\usepackage{mathtools}
\usepackage{mdwlist}
\usepackage{anysize}
\usepackage{pslatex}
\usepackage{wrapfig}
\usepackage{scrhack}
\usepackage{csquotes}
\usepackage{xkeyval}
\usepackage{xfor}
\usepackage{amsgen}
\usepackage{xfrac}
\usepackage{fixmath}
...
...
\begin{document}
...
...
\include{Datei-Kapitel-02}
...
...

\chapter{Kapitel 2}
...
...
$  a = \dfrac{1 + cos\eta}{2}  $
...
...
\end{document} 

$  a = \dfrac{1 + cos\eta}{2}  $