%% 
%%  Ein Beispiel der DANTE-Edition
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%%  2. Auflage
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%%  Beispiel 17-03-3 auf Seite 845.
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%%  Copyright (C) 2016 Herbert Voss
%% 
%%  It may be distributed and/or modified under the conditions
%%  of the LaTeX Project Public License, either version 1.3
%%  of this license or (at your option) any later version.
%% 
%%  See http://www.latex-project.org/lppl.txt for details.
%% 
%% 
%% ==== 
% Show page(s) 1
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\documentclass[]{exaarticle}
\pagestyle{empty}
\setlength\textwidth{352.81416pt}
\setlength\parindent{0pt}
\usepackage[utf8]{inputenc}
%StartShownPreambleCommands
\usepackage{xparse,siunitx}
\ExplSyntaxOn
\NewDocumentCommand { \calcnum } { m }{ \num { \fp_to_scientific:n {#1} } }
\ExplSyntaxOff
%StopShownPreambleCommands
\begin{document}
$\calcnum{ 2 pi * sin ( 2.3 ^ 5 ) + tan ( pi/4 )} \approx
 \calcnum{round( 2 pi * sin ( 2.3 ^ 5 ) + tan ( pi/4 ),3) }$
\end{document}