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Proin quis (tortor) orci. Etiam at risus et $(a + b)x$.
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documentclass{article}
usepackage{fontspec}
usepackage{unicode-math}
setmathfont[range="0028]{Georgia}
setmainfont{Arial}
begin{document}
Proin quis (tortor) orci. Etiam at risus et $(a + b)x$.
end{document}