\documentclass[12pt]{article}
\pagestyle{empty}
\begin{document}
\begin{displaymath}
p_{\rm trial}^{\rm arb}(\theta_{\rm g_a,f,p}) =
{
{{\rm Sin}\left(\theta_{\rm g_a,f,p}\right)}
\over
{{\rm Cos}\left(\theta_{\rm low}\right) - {\rm Cos}\left(\theta_{\rm high}\right)
}
}
\end{displaymath}
\end{document}