\documentclass{article}
\usepackage{amsmath,amssymb}
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\begin{document}
$$
 \begin{bmatrix}
0&\dfrac{1}{a}&0\\\dfrac{2}{5}&-\dfrac{3}{4\,a}&-\dfrac{1}{20}\\-\dfrac{1}{5}&\dfrac{1}{4\,a}&\dfrac{3}{20}
\end{bmatrix}$$
\end{document}
