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\usepackage[latin1]{inputenc}
\usepackage{amsmath}
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\RequirePackage{amsmath,amssymb,latexsym}
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\begin{document}
$  \dfrac{x-x_{a}}{x_{b}-x_{a}}=\dfrac{y-y_{a}}{y_{b}-xy_{a}}=\dfrac{z-z_{a}}{z_{b}-z_{a}}  $
\end{document}
