\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath}
\pagestyle{empty}
\begin{document}
\begin{equation*}P_X(x\neq
1)=\frac{1-2^{-\frac{n}{4}}}{2^n-1}\end{equation*}
\end{document}