\documentclass[a4paper, 12pt]{article}
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\begin{document}
\begin {equation}$$$
\underbrace{ \frac{a_{0}}{2} \underbrace{ \int \limits_{0}^{2 \pi } \cos (m x) dx }_{0}}{0}+\sum_{n=1}^{\infty} \left[\underbrace{ a_{n}\cdot \underbrace{ \int \limits_{0}^{2 \pi } \cos (n x) \cdot \cos(m x) dx}{0| \pi}+ b_{n}\cdot \underbrace{ \int \limits_{0}^{2 \pi } \sin (n x) \cdot \cos (m x) dx}{0|0}}{0|a_{n}\pi }\right] $$$
\end{equation}
\end{document}
Anna
