1

How can I center the columns of the inequallity, so that they are covered under the same \underbrace?

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{alignat}{4}
\left[\sigma_n- \tfrac{1}{2} (\sigma_2 + \sigma_3) \right]^2 &+ \tau_n^2 &\ge \left[ \tfrac{1}{2}(\sigma_2 - \sigma_3) \right]^2 &\qquad\text{(ver circunferencia $C_1$)}\label{eq:MohrC1}\\ 
\left[\sigma_n- \tfrac{1}{2} (\sigma_1 + \sigma_3) \right]^2 &+ \tau_n^2 &\le \left[ \tfrac{1}{2}(\sigma_1 - \sigma_3) \right]^2 &\qquad\text{(ver circunferencia $C_2$)} \label{eq:MohrC2}\\
\underbrace{\left[ \sigma_n- \tfrac{1}{2} (\sigma_1 + \sigma_2) \right]^2}_{\left(x-x_0\right)^2} &+ \underbrace{\tau_n^2}_{\left(y-y_0\right)^2} &\ge \underbrace{\left[ \tfrac{1}{2}(\sigma_1 - \sigma_2) \right]^2}_{r^2} &\qquad\text{(ver circunferencia $C_3$)} \label{eq:MohrC3}
\end{align}
\end{document}

enter image description here

3

This required several things in addition to ending an alignat with an alignat, not an align.

I first started by placing the middle underbrace with a \mathclap, so that the overspilling underbrace would not affect spacing. However, I found that the brace itself still affected the spacing, so I got rid of that and instead used a stack with the \useanchorwidth parameter (argument 7 of \stackengine) as T. This meant that the complete understack would not affect the spacing. In essence, I used a \phantom of the main term to obtain the horizontal spacing, and placed the complete underbraced term as the understack (taking zero width, when \useanchorwidth is set True).

I finally need to add some empty groups to get proper math spacing around the inequalities.

\documentclass{article}
\usepackage{mathtools,stackengine}
\stackMath
\begin{document}
\begin{alignat}{4}
\left[\sigma_n- \tfrac{1}{2} (\sigma_2 + \sigma_3) \right]^2 &+ \tau_n^2 
  &{}\ge \left[ \tfrac{1}{2}(\sigma_2 - \sigma_3) \right]^2 &\qquad
  \text{(ver circunferencia $C_1$)}\label{eq:MohrC1}\\ 
\left[\sigma_n- \tfrac{1}{2} (\sigma_1 + \sigma_3) \right]^2 &+ \tau_n^2 
  &{}\le \left[ \tfrac{1}{2}(\sigma_1 - \sigma_3) \right]^2 &\qquad
  \text{(ver circunferencia $C_2$)} \label{eq:MohrC2}\\
\underbrace{\left[ \sigma_n- \tfrac{1}{2} (\sigma_1 + \sigma_2) 
  \right]^2}_{\left(x-x_0\right)^2} &+ 
  \stackengine{0pt}{\phantom{\tau_n^2}}
  {\underbrace{\tau_n^2}_{\left(y-y_0\right)^2}}{U}{c}{F}{T}{L} 
  &{}\ge \underbrace{\left[ \tfrac{1}{2}(\sigma_1 - \sigma_2) 
  \right]^2}_{r^2} &\qquad\text{(ver circunferencia $C_3$)} 
\label{eq:MohrC3}
\end{alignat}
\end{document}

enter image description here

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