上下に括弧

\begin{equation}
\overbrace{a+b}
\end{equation}
\begin{equation}
\overbrace{a+\cdots +z}^{26}
\end{equation}
\begin{equation}
\underbrace{a+b}
\end{equation}
\begin{equation}
\underbrace{a+\cdots +a}_{n個}
\end{equation}