Danke schon mal



\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[Pr(x_{i}\mid \chi_{i})=
\begin{cases}
\frac{1}{2}+\frac{1}{2}\tanh \frac{\chi_i}{2} &,x_i=+1;\\
\frac{1}{2}-\frac{1}{2}\tanh \frac{\chi_i}{2} &,x_i=-1;
\end{cases}\]
\begin{equation}
Formel + Nummer = Formelnummer :)
\end{equation}
\end{document}
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
Pr(x_{i}\mid \chi_{i})=
\begin{cases}
\frac{1}{2}+\frac{1}{2}\tanh \frac{\chi_i}{2} &,x_i=+1;\\
\frac{1}{2}-\frac{1}{2}\tanh \frac{\chi_i}{2} &,x_i=-1;
\end{cases}
\end{equation}
\begin{equation}
Formel + Nummer = Formelnummer
\end{equation}
\end{document}