\bigg 不适用于左括号

Question

\documentclass[11pt]{article}
\usepackage{xfrac, array, tabu, multirow, graphicx, setspace, dcolumn, tikz}
\usepackage[fleqn]{mathtools}
%the argument for d specifies the maximum number of decimal places
\usepackage[ngerman]{babel}
\usepackage[applemac]{inputenc}
\begin{document}

\begin{align}
x^2&=\sum_{x=1}^{30} \bigg((f_{obs}-f_{exp}\bigg)^2/f_{exp}\\
x^2&=\underline{10.55}
\end{align}
\end{document}

我似乎找不到错误，我遗漏了什么？