答案1
所以我这样做了并且成功了
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{displaymath}
\hat F =
\frac
{\sum_{k=1}^{N}\bigl(\hat x_{k + 1} - \hat F \hat x _k \bigr)}
{\sum_{k=1}^{N} \hat x ^{2}_k}
\end{displaymath}
\end{document}