How to put a big bracket under different parts of a equation so that I could write e.g. a text specific to that part in the equation?

Question

How to put a big bracket under different parts of a equation so that I could write e.g. a text specific to that part in the equation?

For example :

f =  x^3 +  2
    |___|  |__| 
      |     |
   text 1  text 2

Answer 1

What you want is

f = \underbrace{x^3}_\textrm{text 1} + \underbrace{2}_\textrm{text 2}

Answer 2

You would find it out in the mathmode manual or symbols manual.

Answer 3

Since the text used in \underbrace (or \overbrace) is typically set in a different font size to not distract further from the equation, it is preferable to use amsmath's \text macro.

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\[
  f(x) = \underbrace{(x+2)^3}_\text{text 1} + \overbrace{(x-3)}^\text{text 2}
\]
\end{document}