n進数から10進数への変換式をlatexで作成．
test.tex
\documentclass{article}

\usepackage{xstring}
\usepackage{xifthen}
\usepackage{pgf}

\usepackage{amsmath} % align

\begin{document}

\newcounter{ep}
\newcounter{index}

\newcommand{\toD}[2][2]{
   \StrLen{#2}[\mystrlen]
   \setcounter{ep}{\mystrlen}
   \setcounter{index}{0}%
   \whiledo{\value{index} < \mystrlen}{%
      \addtocounter{ep}{-1}
      \stepcounter{index}%
      \StrChar{#2}{\theindex} \times #1^\theep
      \ifnum\theep>0{{}+{}}\fi
   }
}
\newcommand{\toDD}[2][2]{
   \StrLen{#2}[\mystrlen]
   \setcounter{ep}{\mystrlen}
   \setcounter{index}{0}%
   \whiledo{\value{index} < \mystrlen}{%
      \addtocounter{ep}{-1}
      \stepcounter{index}%
      \StrChar{#2}{\theindex} \times 
      \pgfmathint{#1^\theep}\pgfmathresult
      \ifnum\theep>0{{}+{}}\fi
   }
}

\begin{align}
(0334)_8 &= \toD[8]{0334} \\
         &= \toDD[8]{0334} \\
%
(1011011)_2 &= \toD{1011011} \\
            &= \toDD{1011011}
\end{align}

\end{document}
こうなります．