\input amstex

\def\dd{\hbox{d}}

$$\underbrace{\int_{t_0}^t\dotsb\int_{t_0}^t}_{n}f(\xi)
\underbrace{\dd\xi\dotsb\dd\xi}_{n}=\frac{1}{(n-1)!}
\int_{t_0}^tf(\xi)(t-\xi)^{n-1}\,\dd\xi$$
\bye